| Oasis Centre des femmes | Toronto |
| OCD North | Barrie |
| OCD Wellness | Barrie |
| Ombudsman Ontario | Toronto |
| Ontario Aboriginal Housing Services | Sault Ste. Marie |
| Ontario Addiction Treatment Centres, Alliston Office | New Tecumseth |
| Ontario Association of Optometrists, Eye See...Eye Learn | Toronto Downtown Central |
| Ontario College Application Service | Guelph |
| Ontario College of Trades | Toronto Downtown |
| Ontario Dental Association | Toronto Downtown North |
| Ontario Energy Board, Ontario Electricity Support Program (OESP) | Ontario |
| Ontario English Catholic Teachers Association. Simcoe Muskoka Unit | Barrie |
| Ontario Federation of Snowmobile Clubs, Mailing Address | Barrie |
| Ontario Genealogical Society, Simcoe County Branch - Mailing Address | Barrie |
| Ontario Health - Trillium Gift of Life Network | Toronto Downtown Central |
| Ontario Health atHome, Barrie - Sperling Dr | Barrie |
| Ontario Health atHome, Barrie - Sperling Dr, Telehomecare | Barrie |
| Ontario Health atHome, No physical location, Reimbursement for Eligible Expenses for Medical Supplies | Barrie |
| Ontario Hooking Craft Guild, Mailing Address | Minesing |
| Ontario Human Rights Commission, 180 Dundas St W | Toronto Downtown |
| Ontario Incontinence Supplies | New Tecumseth |
| Ontario Motor Vehicle Industry Council | Toronto |
| Ontario Nature | Toronto Downtown |
| Ontario Parents Advocating for Children with Cancer | Barrie |
| Ontario Photo Card | Ontario |
| Ontario Poison Centre | Toronto Downtown Central |
| Ontario Provincial Police, Alliston, Nottawasaga Detachment | New Tecumseth |
| Ontario Provincial Police, Wasaga Beach, Huronia West Detachment | Wasaga Beach |
| Ontario Registered Music Teachers Association, Barrie Branch, No Physical Address | Barrie |
| Ontario Secondary School Teachers' Federation, Barrie, Simcoe County District 17 | Barrie |
| Ontario SPCA and Humane Society, Barrie - 91 Patterson Rd, Ontario SPCA - Barrie Animal Centre | Barrie |
| Ontario SPCA and Humane Society, Barrie - 91A Patterson Rd, Marion Vernon Memorial Animal Clinic | Barrie |
| Ontario Track3 Adaptive Sports Association, Toronto - Administrative Office | Toronto |
| Ontario Vipassana Centre, Egbert, Dhamma Torana | Egbert |
| Ontario Water Centre (The) | Georgina |
| Ontario, Ministry of the Solicitor General, Barrie, Probation and Parole Services, Barrie | Barrie |
| Ontario, Ministry of the Solicitor General, Southwest Region - Midhurst, The Office of the Fire Marshal (OFM) | Springwater |
| Ontario. Members of Provincial Parliament, Barrie-Innisfil Riding, Provincial Constituency Office - Andrea Khanjin | Barrie |
| Ontario. Members of Provincial Parliament, Provincial Constituency Office - Brian Saunderson, Simcoe-Grey Riding - Stayner Office, Simcoe-Grey Riding - Stayner Office | Stayner |
| Ontario. Members of Provincial Parliament, Simcoe-Grey Riding - Alliston Office, Provincial Constituency Office - Brian Saunderson, Simcoe-Grey Riding - Alliston Office | Alliston |
| Ontario. Members of Provincial Parliament, York-Simcoe Riding, Provincial Constituency Office - Caroline Mulroney | East Gwillimbury |
| Ontario. Ministry of Advanced Education and Skills Development, Employment Ontario Hotline | Toronto Downtown |
| Ontario. Ministry of Children, Community and Social Services | Barrie |
| Ontario. Ministry of Children, Community and Social Services, Barrie - 34 Simcoe St, Ontario Disability Support Program - Employment Support Program | Barrie |
| Ontario. Ministry of Children, Community and Social Services, Barrie - 34 Simcoe St, Ontario Disability Support Program | Barrie |
| Ontario. Ministry of Children, Community and Social Services, Barrie - Bayfield St, Youth Justice Division | Barrie |
| Ontario. Ministry of Children, Community and Social Services, Barrie - Ferris Lane, Passport Program | Barrie |
| Ontario. Ministry of Children, Community and Social Services, Newmarket Office, Special Services at Home and Assistance for Children with Severe Disabilities | Newmarket |
| Ontario. Ministry of Children, Community and Social Services, Post Adoption Disclosure Unit | Toronto |
| Ontario. Ministry of Children, Community and Social Services. Family Responsibility Office | Toronto |
| Ontario. Ministry of Children, Community and Social Services. Victim Support Line | Toronto |
| Ontario. Ministry of Economic Development and Growth, Simcoe Muskoka Office | Springwater |
| Ontario. Ministry of Economic Development, Trade and Employment/Ministry of Research and Innovation, 56 Wellesley St. W, Entrepeneur Branch Summer Company Program | Toronto Downtown |
| Ontario. Ministry of Education, Central East Region, Child Care Quality Assurance and Licensing | Toronto |
| Ontario. Ministry of Environment, Conservation and Parks | Barrie |
| Ontario. Ministry of Environment, Conservation and Parks, Aurora District Office, Holland Marsh Provincial Wildlife Management Area | Bradford West Gwillimbury |
| Ontario. Ministry of Environment, Conservation and Parks, Thornbury, Niagara Escarpment Commission | Blue Mountains |
| Ontario. Ministry of Finance., Oshawa, Retail Sales Tax | Oshawa |
| Ontario. Ministry of Government and Consumer Services. Consumer Protection Ontario | Toronto |
| Ontario. Ministry of Health, Trillium Drug Program | Etobicoke |
| Ontario. Ministry of Health, Health Care Connect | Toronto Downtown Central |
| Ontario. Ministry of Health, Health811 | Toronto Downtown Central |
| Ontario. Ministry of Health, Ontario Health Insurance Plan | Toronto Downtown Central |
| Ontario. Ministry of Health, Dental Care For Low-Income Seniors | Toronto |
| Ontario. Ministry of Labour, Immigration, Training and Skills Development [Central East], Barrie Regional Office, Apprenticeship Office, Barrie Regional Office | Barrie |
| Ontario. Ministry of Labour, Immigration, Training and Skills Development. Employment Practices Branch | Toronto |
| Ontario. Ministry of Labour, Immigration, Training and Skills Development. Employment Practices Branch, Newmarket, Ontario. Ministry of Labour, Immigration, Training and Skills Development. Employment Practices Branch, Central Region West, Newmarket | Newmarket |
| Ontario. Ministry of Municipal Affairs and Housing. Rental Housing Enforcement Unit | Toronto |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - 5th Floor, Crown Attorney | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - 5th Floor, Justice of the Peace - Criminal Matters | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - 5th Floor, Victim / Witness Assistance Program | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Office 301, Enforcement and Finance - Sheriff's Office | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Office 301, Estates Administration | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Office 301, Jury Line | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Office 301, Superior Court of Justice - Civil Filing Office | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Office 301, Superior Court of Justice - Family Court | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Office 301, Superior Court of Justice - Small Claims Court | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Room 303, Family Law Information Centre | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Room 313, Manager of Court Operations Office | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Room 412, Superior Court of Justice - Trial Coordinators Office | Barrie |
| Ontario. Ministry of the Attorney General, Barrie Courthouse - Room 501, Ontario Court of Justice - Criminal Court | Barrie |
| Ontario. Ministry of the Attorney General, Toronto, Office for Victims of Crime | Toronto Downtown |
| Ontario. Ministry of the Attorney General. Justice Ontario, Toronto | Toronto |
| Ontario. Ministry of the Attorney General. Office of the Public Guardian and Trustee | Toronto |
| Ontario. Ministry of the Solicitor General, Alliston Office, Probation and Parole Services, Alliston Office | New Tecumseth |
| Ontario. Ministry of the Solicitor General, Penetanguishene, Central North Correctional Centre | Penetanguishene |
| Ontario. Ministry of Transportation, Barrie - Commerce Park Dr, Central Region - International Registration Plan | Barrie |
| Open Arms Early Childhood Centre, Barrie, Child Care Program | Barrie |
| Operation Christmas Child, Simcoe West | Barrie |
| Operation Ukrainian Safe Haven | |
| OPP Chorus | Barrie |
| Orillia Soldiers' Memorial Hospital, Children's Therapy Services, Orillia - 170 Colborne St W | Orillia |
| Orillia Soldiers' Memorial Hospital, North Simcoe Muskoka Regional Genetics Program, Orillia - 170 Colborne St W | Orillia |
| Orillia Soldiers' Memorial Hospital, Regional Domestic Violence Treatment Centre, Orillia - 170 Colborne St W | Orillia |
| Orillia Soldiers' Memorial Hospital, Sexual Assault Treatment Centre, Orillia - 170 Colborne St W | Orillia |
| Orillia Soldiers' Memorial Hospital, Speech Language Pathology, Orillia - 170 Colborne St W | Orillia |
| Oro-Medonte Township, Oro-Medonte - Line 7 S, Municipal Office | Oro-Medonte |
| Our Town Food Bank | Tottenham |
| Ovarian Cancer Canada | Toronto Downtown |
| Overall Independence, Mailing Address | Barrie |
| Overeaters Anonymous, Mailing Address, Central Ontario Intergroup | Waterloo |
| Oxford College, Bayfield Mall | Barrie |