| | Organization Name(s) | Located In |
 | Oak Centre | Welland |
 | Oak Centre, Lake House, Port Colborne | Port Colborne |
 | Oak Centre, Transitional Employment | Welland |
 | Oakwood Park Lodge | Niagara Falls |
 | Obsessive-Compulsive Disorder Self Help Group | St Catharines |
 | Occasions in Niagara | Niagara Falls |
 | Occupational Health Clinics for Ontario Workers | Hamilton East |
| | Offender Reintegration and Assistance Program | St Catharines |
 | Office of the Independent Police Review Director | Toronto |
 | Office of the Ontario Ombudsman | Toronto (City of) |
 | Office of the Privacy Commissioner of Canada | Ottawa |
 | Office of the Provincial Advocate for Children and Youth | Toronto (City of) |
| | OmbudService for Life and Health Insurance | Toronto (City of) |
 | Ombudsman for Banking Services and Investments | Toronto (City of) |
| | ONESource, Moving Solutions for Seniors | St Catharines |
 | Ontario Accommodation Association | Peterborough |
 | Ontario Addiction Treatment Centres, Niagara Falls Clinic | Niagara Falls |
| | Ontario Association for Marriage and Family Therapy | Tottenham |
 | Ontario Association of Non-Profit Homes and Services for Seniors | Woodbridge |
| | Ontario Association of Social Workers | St Catharines |
 | Ontario Brain Injury Association | Thorold |
 | Ontario Camps Association | Toronto (City of) |
 | Ontario College of Pharmacists | Toronto (City of) |
 | Ontario Dental Association (ODA) | Toronto (City of) |
 | Ontario Early Years Centre, Early Years Toy Lending Library | Welland |
 | Ontario Early Years Centre, Erie-Lincoln, Fort Erie-Crystal Ridge Satellite Site | Crystal Beach |
 | Ontario Early Years Centre, Erie-Lincoln, Lincoln Satellite Site, Port Cares | Beamsville |
 | Ontario Early Years Centre, Erie-Lincoln, Port Cares, Main Office & Port Colborne Ontario Early Years Centre | Port Colborne |
 | Ontario Early Years Centre, Erie-Lincoln, Port Colborne Site | Port Colborne |
 | Ontario Early Years Centre, Erie-Lincoln, West Lincoln Satellite Site, Port Cares | Smithville |
 | Ontario Early Years Centre, Niagara Centre Service Area, Early Years Mobile Program | Welland |
 | Ontario Early Years Centre, Niagara Centre Service Area, Fenwick Satellite Site | Fenwick |
 | Ontario Early Years Centre, Niagara Centre Service Area, Grimsby Ontario Early Years Satellite Site - Mountain | Grimsby |
 | Ontario Early Years Centre, Niagara Centre Service Area, Main Hub Site | Welland |
 | Ontario Early Years Centre, Niagara Centre Service Area, West Niagara Family Centre Satellite Site, Grimsby Beach Hall Site | Grimsby |
 | Ontario Early Years Centre, Niagara Falls, Father Hennepin Catholic School | Niagara Falls |
 | Ontario Early Years Centre, Niagara Falls, MacBain Community Centre | Niagara Falls |
 | Ontario Early Years Centre, Niagara Falls, Mary Ward Catholic School, Main Site | Niagara Falls |
| | Ontario Early Years Centre, Niagara Falls, Mobile Resource Program | Niagara Falls |
 | Ontario Early Years Centre, Niagara Falls, Sacred Heart Catholic School | Chippawa |
 | Ontario Early Years Centre, Niagara Falls, St Patrick's Catholic School | Niagara Falls |
 | Ontario Early Years Centre, St Catharines, Bunting Rd Site | St Catharines |
 | Ontario Early Years Centre, St Catharines, Grantham Ave Site | St Catharines |
 | Ontario Early Years Centre, St Catharines, Main Hub Site | St Catharines |
 | Ontario Early Years Centre, St Catharines, Mobile Resource Program | St Catharines |
 | Ontario Early Years Centre, St Catharines, Thorold Site | Thorold |
 | Ontario Energy Board | Toronto (City of) |
 | Ontario Federation of Labour | Toronto (City of) |
| | Ontario Federation of Teaching Parents, St Catharines-Niagara Area | St Catharines |
 | Ontario Genealogical Society, Niagara Peninsula Branch | Thorold |
| | Ontario Horticultural Association, Fort Erie Horticultural Society, Down to Earth Gardeners | Fort Erie |
 | Ontario Horticultural Association, Grimsby Garden Club | Grimsby |
| | Ontario Horticultural Association, Lincoln Garden Club and Horticultural Society | Jordan |
| | Ontario Horticultural Association, Niagara Falls Horticultural Society | Niagara Falls |
 | Ontario Horticultural Association, Niagara on the Lake Horticultural Society | Niagara-on-the-Lake |
| | Ontario Horticultural Association, Port Colborne Horticultural Society | Port Colborne |
| | Ontario Horticultural Association, Smithville Garden Club | West Lincoln |
| | Ontario Horticultural Association, St Catharines Horticultural Society | St Catharines |
 | Ontario Horticultural Association, Thorold Horticultural Society | Thorold |
| | Ontario Horticultural Association, Welland Horticultural Society | Welland |
 | Ontario Human Rights Commission | Toronto (City of) |
| | Ontario Independent Facilitation Network, www.oifn.ca | Ontario |
| | Ontario Independent Facilitation Organizations, www.independentfacilitation.ca/map/ | Ontario |
| | Ontario Judicial Council | Toronto |
 | Ontario Lacrosse Hall of Fame and Museum | St Catharines |
 | Ontario Lung Association (The), Hamilton Branch, Breathworks Program | Hamilton East |
| | Ontario Medical Association, Greater Niagara Medical Society | Niagara Falls |
| | Ontario Medical Association, Lincoln County Academy of Medicine | St Catharines |
| | Ontario Medical Association, Niagara South Medical Society, Niagara South Branch | Port Colborne |
 | Ontario Motor Vehicle Industry Council | Toronto (City of) |
 | Ontario Natural Food Co-op | Mississauga |
 | Ontario Nature | Toronto (City of) |
 | Ontario Network for the Prevention of Elder Abuse (The) | Toronto |
 | Ontario Office Works Inc | Welland |
 | Ontario Patient Transfer (OPT) | Hamilton Central |
 | Ontario Pension Board, Client Services | Toronto (City of) |
| | Ontario Poison Centre | Toronto (City of) |
| | Ontario Power Authority, Refrigerator Round-Up | Ontario |
| | Ontario Power Authority, Residential Programs | Ontario |
 | Ontario Prader-Willi Syndrome Association | Toronto (City of) |
 | Ontario Provincial Police, Niagara Detachment | Niagara Falls |
 | Ontario Psychological Association | Toronto (City of) |
 | Ontario Psychotherapy and Counseling Referral Network | St Catharines |
 | Ontario Psychotherapy Counselling and Associates | St Catharines |
 | Ontario Public Service Employees Union | Fonthill |
| | Ontario Registered Music Teachers Association, Niagara Falls Branch | Niagara Falls |
 | Ontario Retirement Communities Association | Oakville |
 | Ontario Science Centre | North York |
 | Ontario Students Against Impaired Driving (OSAID) | Toronto |
| | Ontario Tenants Rights | Ontario |
 | Ontario Treatment Rehabilitation and Counselling (ONTRAC) | St Catharines |
 | Ontario Trillium Foundation, Niagara Office | St Catharines |
| | Ontario Veterinary Medical Association, Safe Pet Program | Milton |
| | Ontario Wine Society, The | Vineland |
 | Ontario. Employment Ontario, Apprenticeship Office, Niagara Region Office | St Catharines |
| | Ontario. Employment Ontario, Employment Ontario Hotline | Ontario |
 | Ontario. Legislative Assembly - Cabinet Ministers | Toronto (City of) |
 | Ontario. Members of the Legislative Assembly, Hamilton Centre Provincial Riding, Andrea Horwath, MPP | Hamilton Central |
 | Ontario. Members of the Legislative Assembly, Niagara Falls Provincial Riding, Kim Craitor, MPP, Fort Erie Office | Fort Erie |
 | Ontario. Members of the Legislative Assembly, Niagara Falls Provincial Riding, Kim Craitor, MPP, Niagara Falls Office | Niagara Falls |
 | Ontario. Members of the Legislative Assembly, Niagara West - Glanbrook Provincial Riding, Tim Hudak, MPP | Beamsville |
 | Ontario. Members of the Legislative Assembly, Office of the Leader of the Opposition - Progressive Conservative Party (PC), Niagara West-Glanbrook Provincial Riding, Tim Hudak, MPP | Toronto (City of) |
 | Ontario. Members of the Legislative Assembly, St Catharines Provincial Riding, Jim Bradley, MPP | St Catharines |
 | Ontario. Members of the Legislative Assembly, Welland Provincial Riding, Cindy Forster, MPP, Port Colborne Office | Port Colborne |
 | Ontario. Members of the Legislative Assembly, Welland Provincial Riding, Cindy Forster, MPP, Thorold Office | Thorold |
 | Ontario. Members of the Legislative Assembly, Welland Provincial Riding, Cindy Forster, MPP, Welland Office | Welland |
 | Ontario. Ministry of Agriculture and Food and the Ministry of Rural Affairs | Vineland Station |
 | Ontario. Ministry of Children and Youth Services, Child and Parent Resource Institute (CPRI) | London |
 | Ontario. Ministry of Children and Youth Services, Ontario Child Benefit (OCB) | Toronto (City of) |
 | Ontario. Ministry of Children and Youth Services, Private and International Adoption Unit | Toronto (City of) |
 | Ontario. Ministry of Children and Youth Services, Youth Justice Services, St Catharines | St Catharines |
 | Ontario. Ministry of Citizenship and Immigration, Regional Services | St Catharines |
| | Ontario. Ministry of Community and Social Services, Family Responsibility Office | Toronto (City of) |
 | Ontario. Ministry of Community and Social Services, Ontario Disability Support Program, St Catharines | St Catharines |
| | Ontario. Ministry of Community and Social Services, Welfare Fraud Unit | Toronto (City of) |
 | Ontario. Ministry of Community and Social Services and Ministry of Children and Youth Services | Hamilton Central |
 | Ontario. Ministry of Community and Social Services and Ministry of Children and Youth Services, Assistance for Children with Severe Disabilities | Hamilton Central |
 | Ontario. Ministry of Community and Social Services and Ministry of Children and Youth Services, Assistance for Children with Severe Disabilities, Special Services at Home | Hamilton Central |
 | Ontario. Ministry of Community Safety and Correctional Services, Niagara Detention Centre | Thorold |
 | Ontario. Ministry of Community Safety and Correctional Services, Office of the Chief Coroner, Regional Supervising Coroners, West Region | Hamilton |
 | Ontario. Ministry of Community Safety and Correctional Services, Ontario Civilian Police Commission | Toronto |
 | Ontario. Ministry of Community Safety and Correctional Services, Probation and Parole Services, Niagara Falls | Niagara Falls |
 | Ontario. Ministry of Community Safety and Correctional Services, Probation and Parole Services, St Catharines | St Catharines |
 | Ontario. Ministry of Community Safety and Correctional Services, Probation and Parole Services, Welland | Welland |
| | Ontario. Ministry of Consumer Services, Consumer Protection Branch | Toronto (City of) |
| | Ontario. Ministry of Consumer Services, Retail Business Holidays Act | Toronto (City of) |
 | Ontario. Ministry of Culture, Ontario Arts Council | Toronto (City of) |
 | Ontario. Ministry of Economic Development, Trade and Employment, Business Advisory Services Branch, Central Region, St Catharines | St Catharines |
| | Ontario. Ministry of Energy, Ontario Clean Energy Benefit | Ontario |
 | Ontario. Ministry of Finance | Oshawa |
 | Ontario. Ministry of Finance, Client Services Branch | Hamilton Central |
| | Ontario. Ministry of Finance, Healthy Homes Renovation Tax Credit | Oshawa |
 | Ontario. Ministry of Finance, Ontario Tax Credits | Oshawa |
 | Ontario. Ministry of Government and Consumer Services, Companies and Personal Property Security Branch, Names and Articles Control Incorporation as a Non-profit Organization | Toronto |
| | Ontario. Ministry of Government Services, Office of the Registrar General, Birth, Death, Marriage Certificates | Thunder Bay |
 | Ontario. Ministry of Government Services, ServiceOntario, INFOline | Toronto (City of) |
| | Ontario. Ministry of Health and Long Term Care, EatRight Ontario | Toronto |
 | Ontario. Ministry of Health and Long-Term Care, Central South Region - Hamilton Office, Performance Improvement and Compliance Branch, Healthy System Accountability and Performance Division | Hamilton Central |
| | Ontario. Ministry of Health and Long-Term Care, Health Care Connect | Toronto |
 | Ontario. Ministry of Health and Long-Term Care, Health Services Appeal and Review Board | Toronto |
| | Ontario. Ministry of Health and Long-Term Care, Healthy Smiles Ontario | Ontario |
| | Ontario. Ministry of Health and Long-Term Care, MedsCheck | Toronto |
 | Ontario. Ministry of Health and Long-Term Care, Niagara Nurse Practitioner-Led Clinic | Niagara Falls |
| | Ontario. Ministry of Health and Long-Term Care, Ontario Drug Benefit | Ontario |
 | Ontario. Ministry of Health and Long-Term Care, Ontario Health Insurance Plan | St Catharines |
| | Ontario. Ministry of Health and Long-Term Care, Operational Support Branch, Assistive Devices Program and Home Oxygen Program | Toronto (City of) |
| | Ontario. Ministry of Health and Long-Term Care, Trillium Drug Program | Toronto (City of) |
 | Ontario. Ministry of Labour, Niagara Regional Office | St Catharines |
 | Ontario. Ministry of Labour, Office of the Worker Advisor | St Catharines |
 | Ontario. Ministry of Labour, Pay Equity Commission, Pay Equity Office | Toronto (City of) |
 | Ontario. Ministry of Municipal Affairs, Landlord and Tenant Board, Regional Office - Hamilton | Hamilton Central |
 | Ontario. Ministry of Natural Resources, Area Office | Vineland |
 | Ontario. Ministry of the Attorney General, Alcohol and Gaming Commission of Ontario | Toronto (City of) |
 | Ontario. Ministry of the Attorney General, Child and Family Services Review Board | Toronto |
 | Ontario. Ministry of the Attorney General, Court Services, Family Law Information Centre, Niagara Falls | Niagara Falls |
 | Ontario. Ministry of the Attorney General, Court Services, Family Law Information Centre, St Catharines | St Catharines |
 | Ontario. Ministry of the Attorney General, Court Services, Family Law Information Centre, Welland | Welland |
 | Ontario. Ministry of the Attorney General, Court Services, St Catharines | St Catharines |
 | Ontario. Ministry of the Attorney General, Court Services, Welland | Welland |
 | Ontario. Ministry of the Attorney General, Crown Attorney, Niagara North | St Catharines |
 | Ontario. Ministry of the Attorney General, Crown Attorney, Niagara South | Welland |
 | Ontario. Ministry of the Attorney General, Office of the Public Guardian and Trustee, Hamilton Branch | Hamilton Central |
 | Ontario. Ministry of the Attorney General, Office of the Public Guardian and Trustee, Hamilton Branch, Guardianship Investigations | Toronto |
 | Ontario. Ministry of the Attorney General, Victims and Vulnerable Persons Division, Office of the Children's Lawyer | Toronto (City of) |
| | Ontario. Ministry of the Attorney General, Victims and Vulnerable Persons Division, Victim Support Line | Toronto |
 | Ontario. Ministry of the Attorney General, Victims and Vulnerable Persons Division, Victim/Witness Assistance Program, St Catharines | St Catharines |
 | Ontario. Ministry of the Attorney General, Victims and Vulnerable Persons Division, Victim/Witness Assistance Program, Welland Satellite Office | Welland |
| | Ontario. Ministry of the Environment, Drive Clean Program, Call Centre | Toronto (City of) |
 | Ontario. Ministry of the Environment, Niagara District Office | St Catharines |
 | Ontario. Ministry of the Environment, Niagara Escarpment Commission, St Catharines Satellite Office | St Catharines |
 | Ontario. Ministry of the Environment, Operations Division, West Central Region | Hamilton Central |
 | Ontario. Ministry of Tourism, Tourism Division, Ontario Travel Information Centre, Niagara Falls | Niagara Falls |
 | Ontario. Ministry of Tourism, Tourism Division, Ontario Travel Information Centre, St Catharines | Niagara-on-the-Lake |
 | Ontario. Ministry of Tourism and Culture, Regional Services | St Catharines |
 | Ontario. Ministry of Tourism, Culture and Sport, Regional Tourism Advisor | Kitchener |
 | Ontario. Ministry of Training, Colleges and Universities, Employment and Training Division, Employment Ontario, Apprenticeship, Second Career | Hamilton Central |
 | Ontario. Ministry of Training, Colleges and Universities, Employment and Training Division, Employment Ontario, Niagara Region | St Catharines |
| | Ontario. Ministry of Training, Colleges and Universities, Employment Ontario | Toronto |
 | Ontario. Ministry of Training, Colleges and Universities, Ontario Student Assistance Program, Student Support Branch | Thunder Bay |
 | Ontario. Ministry of Transportation | St Catharines |
 | Ontario. Ministry of Transportation, Board Room Bookings | St Catharines |
 | Ontario. Ministry of Transportation, Enhanced Driver's Licence (EDL), St Catharines | St Catharines |
 | Ontario. Ministry of Transportation, Hamilton/Niagara Enforcement Office | Stoney Creek |
 | Ontario. Ministry of Transportation, Metrolinx, GO Transit | Toronto (City of) |
| | Ontario. Ministry of Transportation, Senior Driver Renewal Program | Ontario |
 | Ontario. ServiceOntario, Fonthill | Fonthill |
 | Ontario. ServiceOntario, Fort Erie | Fort Erie |
 | Ontario. ServiceOntario, Grimsby | Grimsby |
 | Ontario. ServiceOntario, Land Registry Offices 30 & 59, Niagara North and South | St Catharines |
 | Ontario. ServiceOntario, Niagara Falls North, Thorold Stone Road | Niagara Falls |
 | Ontario. ServiceOntario, Niagara-on-the-Lake | Niagara-on-the-Lake |
 | Ontario. ServiceOntario, Port Colborne | Port Colborne |
| | Ontario. ServiceOntario, Publications | Toronto (City of) |
 | Ontario. ServiceOntario, Smithville | Smithville |
 | Ontario. ServiceOntario, St Catharines | St Catharines |
 | Ontario. ServiceOntario, St Catharines West, Scott Street | St Catharines |
 | Ontario. ServiceOntario, Welland | Welland |
 | Oonuhseh Niagara Native Homes Inc | St Catharines |
 | Open Arms Mission Welland Inc | Welland |
 | Open Arms Mission Welland Inc, Redeemed Goods | Welland |
 | Open Arms of Jesus Outreach Ministry | St Catharines |
| | Opry Niagara Classic Country Music Association | Niagara Falls |
 | Optimist Club of Niagara Falls | Niagara Falls |
 | Optimist Club of Port Colborne | Port Colborne |
 | Optimist Club of St Catharines | St Catharines |
 | Optimist Club of Welland | Welland |
 | Orchard Creek Fellowship, Brethren in Christ | St Catharines |
 | Orchard Park Bible Church | Niagara-on-the-Lake |
 | Orchard View Apartments | Vineland |
 | Orchards Retirement Residence, The | Vineland |
 | Original Fall Guy | Dundas |
 | OSGA 55+ Games (Ontario Senior Games Association), District #24- Niagara Region | Toronto |
| | Osteoporosis Canada, Niagara Chapter | St Catharines |
 | Our Lady of Hungary Roman Catholic Church | Welland |
 | Our Lady of Peace Roman Catholic Church | Niagara Falls |
 | Our Lady of Perpetual Help Roman Catholic Church | St Catharines |
 | Our Lady of the Holy Rosary Roman Catholic Church | Thorold |
 | Our Lady of the Scapular Roman Catholic Church | Niagara Falls |
 | Our Saviour Lutheran Church - LCC | Niagara Falls |
| | Out of the Cold Program, St Catharines | St Catharines |
| | OUTniagara | St Catharines |
| | Overeaters Anonymous, Central Ontario Intergroup | Niagara Region |
 | Oxford Learning, Elementary, Preschool and High School Programs | St Catharines |
 | Oxford Learning, Niagara Falls | Niagara Falls |
 | Oxford Learning, St Catharines | St Catharines |
 | Ozanam Centre | St Catharines |