| | Organization Name(s) | Located In |
| | Oak Ridges and District Minor Hockey Association | Oak Ridges |
| | Oak Ridges Brethren In Christ Church | York Region |
| | Oak Ridges Co-ed Receational Slo-Pitch League, Richmond Hill FridayNite Men's Slo-Pitch | Richmond Hill |
| | Oak Ridges Friends Of The Environment Organization | Richmond Hill |
 | Oak Ridges Italian Seniors Club | Richmond Hill |
| Oak Ridges Men's Recreational Hockey League | Richmond Hill |
| Oak Ridges Moraine Foundation | King City |
| Oak Ridges Moraine Land Trust | King City |
| Oak Ridges Soccer Club | Richmond Hill |
| Oak Ridges Trail Association | King City |
| Oakview Terrace Reception Centre | Richmond Hill |
| Oasis of Faith Worship Centre | Newmarket |
| Observatory Towers | Richmond Hill |
| Office of the Independent Police Review Director | Ontario |
| Office of the Ontario Ombudsman | Toronto |
| OHR Somayach Residential Centre | Thornhill |
| Old Markham Village Ratepayers Inc. | Markham |
| | Older Adult Aquafit | Markham |
| Older Adults In Action | Markham |
| Older Women's Network | Toronto |
| Olive Branch Community Church (The) | Markham |
| Ombudsman for Banking Services and Insurance | Toronto |
| | One of a Kind Canadian Craft Show and Sale | Toronto |
| One of a Kind Show and Sale | Toronto |
| One Parent Families Association of Canada, Richmond Hill Chapter | Richmond Hill |
| | One Silver Lining | Markham |
| | Ontario, Lieutenant Governor of Ontario, The Honourable David C. Onley | Toronto |
| | Ontario, What To Do When Someone Dies | Ontario |
| Ontario Archaeological Society | Toronto |
| | Ontario Association for Marriage and Family Therapy | Tottenham |
| Ontario Association of Consultants, Counsellors, Psychometrists and Psychotherapists | Toronto |
| Ontario Association of Non Profit Homes and Services for Seniors | Woodbridge |
| Ontario Association of Speech Language Pathologists and Audiologists | Toronto |
| Ontario Ball Hockey Association | Vaughan |
| Ontario Camps Association | Toronto |
| Ontario Centre of Excellence for Child and Youth Mental Health | Ottawa |
| Ontario Chiropractic Association | Mississauga |
| Ontario Civilian Commission on Police Services | Toronto |
| Ontario Clean Air Alliance | Toronto |
| Ontario Clean Air Alliance | Toronto |
| Ontario College of Pharmacists | Toronto |
| Ontario College of Teachers | Toronto |
| Ontario Criminal Injuries Compensation Board | Toronto |
| Ontario Cross-Cultural Music Society | Richmond Hill |
| Ontario Dental Association | Toronto |
| Ontario Disability Support Program | Newmarket |
| Ontario Early Years Centre | Richmond Hill |
| Ontario Early Years Centre, Markham | Markham |
| Ontario Early Years Centre, Vaughan-King-Aurora Riding, Aurora Satellite | Aurora |
| | Ontario Environmental Network | North Bay |
| Ontario Farmland Trust | Guelph |
| Ontario Federation for Cerebral Palsy OFCP | Toronto |
| | Ontario Genealogical Society York Region Branch | Richmond Hill |
| Ontario Gerontology Association, OGA/AOG | Toronto |
| | Ontario Government | Ontario |
| Ontario Greenbelt Alliance | Toronto |
| Ontario Heritage Trust | Toronto |
| Ontario Heritage Trust Week- Toronto | Toronto |
| Ontario Human Rights Commission | Toronto |
| Ontario Liberal Party | Toronto |
| Ontario March of Dimes | Richmond Hill |
| Ontario March of Dimes, Independent Living Support Services | Richmond Hill |
| Ontario March of Dimes, Stroke Recovery Support Group, York Region | Richmond Hill |
| Ontario March of Dimes, Strategic Employment Solutions | Richmond Hill |
| Ontario Medical Association | Toronto |
| Ontario Ministry of Community and Social Services, Ontario Disability Support Program - Employment Supports | Newmarket |
| Ontario Ministry of Government Services, Consumer Protection Branch, Consumer Services Bureau | Toronto |
| | Ontario Ministry of Training, Colleges and Universities, Employment Ontario | Greater Toronto Area |
| Ontario Motor Vehicle Industry Council | Toronto |
| Ontario Municipal Board | Toronto |
| Ontario Nature | Toronto |
| Ontario Network for the Prevention of Elder Abuse | Toronto |
| Ontario Network of Injured Workers Group | Hamilton |
| Ontario New Democratic Party (The) | Toronto |
| Ontario Nurses Association | Toronto |
| Ontario Obsessive Compulsive Disorder Network | Markham |
| Ontario Obsessive Compulsive Disorder Network (The) | Barrie |
| | Ontario One Call, Call Before You Dig, Cable Locating Service | Ontario |
| Ontario Parks Association | Milton |
| Ontario PC Party | Toronto |
| | Ontario Poison Centre | Toronto |
| Ontario Power Authority, Recycle old refrigerators, Powerstream | York Region |
| Ontario Power Generation, Nuclear Power Station | Toronto |
| Ontario Press Council | Toronto |
| | Ontario Problem Gambling Helpline | Ontario |
| Ontario Provincial Police, Aurora Detachment | Aurora |
| Ontario Psychological Association | Toronto |
| Ontario Real Estate Association (The) | Toronto |
| Ontario Registered Music Teachers Association, Greater Toronto Branch | Toronto |
| | Ontario Registered Music Teachers' Association, Newmarket & Area Branch | Newmarket |
| | Ontario Registered Music Teachers' Association, Richmond Hill | Richmond Hill |
| Ontario Safety Association for Community and Healthcare | Toronto |
| Ontario Safety League, Operation Red Nose | Mississauga |
| Ontario Science Centre | Toronto |
| Ontario Shores Centre for Mental Health Sciences | Durham Region |
| Ontario Society for the Prevention of Cruelty to Animals, OSPCA | Newmarket |
| Ontario Society of Psychotherapists | Toronto |
| Ontario SPCA Provincial Education & Animal Centre, Newmarket | Newmarket |
| Ontario SPCA Provincial Education and Animal Centre | Newmarket |
| Ontario Streams | Aurora |
| | Ontario Tenants Rights | Ontario |
| Ontario Tourism, Ontario Tourism Marketing Partnership Corporation | Toronto |
| | Ontario Trail Riders Association | Markham |
| Ontario Trillium Foundation | Toronto |
| Ontario Wheelchair Sports Association | Toronto |
| | Ontario Wildlife Foundation | Peterborough |
| Ontario Works, Vaughan Branch | Vaughan |
| Ontario York Region Heritage Lions Club | Markham |
| | Ontario, Ministry of Health and Long Term Care, Telephone Health Advisory Service | Ontario |
| Ontario. Information and Privacy Commissioner | Toronto |
| Ontario. Landlord and Tenant Board | Toronto |
| Ontario. Legislative Offices, Premier Dalton McGuinty, Liberal Party of Ontario Leader | Toronto |
| | Ontario. Lieutenant Governor, David C Onley | Toronto |
| Ontario. Members of Provincial Parliament, Markham-Unionville - Michael Chan MPP | Markham |
| Ontario. Members of Provincial Parliament, Newmarket/Aurora, Frank Klees, MPP | Aurora |
| Ontario. Members of Provincial Parliament, Oak Ridges-Markham - Dr Helena Jaczek MPP | Markham |
| Ontario. Members of Provincial Parliament, Richmond Hill Riding, Reza Moridi, MPP | Richmond Hill |
| Ontario. Members of Provincial Parliament, Thornhill - Peter Shurman MPP | Thornhill |
| | Ontario. Ministry of Agriculture, Food and Rural Affairs | Toronto |
| Ontario. Ministry of Attorney General, Child Support Inquiry Line of Ontario, Child Support Guidelines | Oshawa |
| Ontario. Ministry of Children and Youth Services, Licensed Child Care | Markham |
| Ontario. Ministry of Citizenship and Immigration, Seniors and Disabilities, Office for Seniors' Issues - Inquiry Line | Toronto |
| Ontario. Ministry of Community and Social Services, Children and Youth Services, Juvenile Justice Volunteer Program | Newmarket |
| | Ontario. Ministry of Community and Social Services, Family Responsibility Office | Toronto |
| Ontario. Ministry of Community and Social Services, Social Services Division, Community Services and Housing Department, Ontario Works | Richmond Hill |
| Ontario. Ministry of Community Safety and Correctional Services | Greater Toronto Area |
| Ontario. Ministry of Community Safety and Correctional Services, Operations Division, Probation & Parole Services | Richmond Hill |
| Ontario. Ministry of Culture, Ontario Heritage Trust | Toronto |
| Ontario. Ministry of Culture, Ontario Lottery Corporation | Toronto |
| | Ontario. Ministry of Culture, Ontario Seniors' Secretariat | Toronto |
| Ontario. Ministry of Education, Barrie Regional Office | Toronto |
| Ontario. Ministry of Finance | Oshawa |
| Ontario. Ministry of Finance, Financial Services Commission of Ontario | Toronto |
| Ontario. Ministry of Finance, Income Tax Related Programs | Oshawa |
| Ontario. Ministry of Finance, Income Tax Related Programs Branch | Oshawa |
| Ontario. Ministry of Finance, Ontario Child Care Supplement for Working Families, Income Tax Related Programs Branch | Oshawa |
| Ontario. Ministry of Finance, Tax Revenue Division, Guaranteed Income and Tax Credit Branch, Guaranteed Annual Income System (GAINS); Ontario Home Owner | Oshawa |
| | Ontario. Ministry of Government and Consumer Service, ServiceOntario | Toronto |
| Ontario. Ministry of Government Services | Toronto |
| Ontario. Ministry of Government Services, General Administrative Services Delivery Branch, Official Documents Services | Toronto |
| Ontario. Ministry of Health and Long-Term Care | Toronto |
| | Ontario. Ministry of Health and Long-Term Care, AIDS and Sexual Health Information Line | Ontario |
| Ontario. Ministry of Health and Long-Term Care, Assistive Devices Program | Toronto |
| Ontario. Ministry of Health and Long-Term Care, Claims Office | Oshawa |
| Ontario. Ministry of Health and Long-Term Care, Drug Programs Branch | Toronto |
| Ontario. Ministry of Health and Long-Term Care, Drug Programs Branch, Ontario Drug Benefit Program | Ontario |
| Ontario. Ministry of Health and Long-Term Care, Fraud Programs | Toronto |
| Ontario. Ministry of Health and Long-Term Care, Health Boards Secretariat | Toronto |
| Ontario. Ministry of Health and Long-Term Care, Health Care Connect | Ontario |
| Ontario. Ministry of Health and Long-Term Care, Health Care Options Medical Services Directory | Ontario |
| | Ontario. Ministry of Health and Long-Term Care, Long-Term Care ACTION Line | Ontario |
| Ontario. Ministry of Health and Long-Term Care, Medscheck | Ontario |
| | Ontario. Ministry of Health and Long-Term Care, Ontario Health Insurance Plan, OHIP Card Replacement or Renewal | |
| | Ontario. Ministry of Health and Long-Term Care, Ontario Wait Times Website | Ontario |
| Ontario. Ministry of Health and Long-Term Care, Telehealth Ontario | Ontario |
| Ontario. Ministry of Labour, EMPLOYMENT STANDARDS BRANCH, EMPLOYMENT PRACTICES BRANCH, FAMILY MEDICAL LEAVE | Toronto |
| Ontario. Ministry of Labour, EMPLOYMENT STANDARDS INFORMATION CENTRE, EMPLOYMENT PRACTICES BRANCH | Toronto |
| Ontario. Ministry of Labour, Occupational Health and Safety Branch, Industrial Health and Safety Branch | York Region |
| Ontario. Ministry of Labour, Pay Equity Commission | Toronto |
| Ontario. Ministry of Labour, Workplace Hazardous Materials Information Systems | Toronto |
| | Ontario. Ministry of Municipal Affairs and Housing, Commercial Tenancy Information Line | Ontario |
| Ontario. Ministry of Natural Resources, Fishing and Hunting Licences | Toronto |
| Ontario. Ministry of Natural Resources, ServiceOntario | Peterborough |
| Ontario. Ministry of Public Safety and Security, OFFICE OF THE CHIEF CORONER | Toronto |
| Ontario. Ministry of Revenue, Employer Health Tax | Oshawa |
| | Ontario. Ministry of the Attorney General, Courts | Toronto |
| Ontario. Ministry of the Attorney General, Office of the Public Guardian and Trustee | Toronto |
| Ontario. Ministry of the Attorney General, Office of the Public Guardian and Trustee, Power of Attorney Kits | Toronto |
| Ontario. Ministry of the Attorney General, Ontario Court (General Division), Small Claims Court Referees | North York |
| Ontario. Ministry of the Attorney General, Provincial Court, Criminal Division, Court House | Newmarket |
| Ontario. Ministry of the Attorney General, Provincial Offences Court, Civil Division, Small Claims Court | Richmond Hill |
| Ontario. Ministry of the Attorney General, Superior Court of Justice (Family Division), Family Court | Newmarket |
| Ontario. Ministry of the Attorney General, Victim Support Line | Toronto |
| Ontario. Ministry of the Environment, Air Quality Information | North York |
| Ontario. Ministry of the Environment, APPROVALS BRANCH - ADMINISTRATION AND RECORDS SERVICES SECTION | Toronto |
| Ontario. Ministry of the Environment, Environmental Monitoring and Reporting Branch | Toronto |
| Ontario. Ministry of the Environment, Laboratory Services Branch | Etobicoke |
| Ontario. Ministry of the Environment, Ontario Drive Clean Program | Markham |
| Ontario. Ministry of Tourism, Culture and Sport, ONTARIO TRAVEL INFORMATION, TOURISM MARKETING BRANCH, CANADIAN TOURISM COMMISSION | Toronto |
| Ontario. Ministry of Training, Colleges and Universities, DEPARTMENT OF EDUCATION, INDEPENDENT LEARNING CENTRE | Toronto |
| Ontario. Ministry of Training, Colleges and Universities, Ontario Student Assistance Program | Thunder Bay |
| Ontario. Ministry of Training, Colleges and Universities, Ontario Youth Apprenticeship Program | Toronto |
| | Ontario. Ministry of Transportation, Driver and Motor Vehicle Licencing Agents, Markham | Markham |
| Ontario. Ministry of Transportation, Driver and Vehicle Licence Issuing Office, Aurora | Aurora |
| Ontario. Ministry of Transportation, Driver and Vehicle Licence Issuing Office, Richmond Hill, Accessible Parking Permits | Richmond Hill |
| Ontario. Ministry of Transportation, DriveTest Centre | Aurora |
| Ontario. Ministry of Transportation, Public Safety Education and Information Division, Customer Services Branch | Toronto (City of) |
| | Ontario. Ministry of Transportation, Senior Drivers Licence Renewal | Ontario |
| Ontario. Ministry of Transportation, Used Vehicle Information Package | Toronto |
| Ontario. ServiceOntario, Aurora, Vehicle and Driver Permits, Health Card Renewals, Organ and Donor Services | Aurora |
| Ontario. ServiceOntario, Driver and Motor Vehicle Licensing Office, Richmond Hill, Health Card Renewal | Richmond Hill |
| | Ontario's Historical Plaques | York Region |
| | Opera York | Thornhill |
| Operation Come Home | Ottawa |
| | Optimist Club of Aurora | Aurora |
| | Optimist Club of Markham | Markham |
| Optimist Club of Newmarket | Newmarket |
| | Optimist Club Richmond Hill | Richmond Hill |
| | Optimist Recreational Hockey League | Aurora |
| Or Hadash Synagogue | Newmarket |
| Or Hamet Sephardic Synagogue | Thornhill |
| Organization for the Rescue of Animals | Mount Albert |
| Osgoode Society for Canadian Legal History | Toronto |
| Osteoporosis Canada | Toronto |
| Our Lady of Grace Catholic Church | Aurora |
| Our Lady of the Annunciation Roman Catholic Church | Richmond Hill |
| Our Lady of the Rosary School Age Program | Concord |
| Our Lady Queen of the World Roman Catholic Church | Richmond Hill |
| Outward Bound Canada | Toronto |
| Ovarian Cancer Canada | Toronto |
| | Overeaters Anonymous | Ontario |
| Oxfam Canada | Toronto |
| Oxford Learning Centre, Richmond Hill Branch | Richmond Hill |
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