| Onehe Limited | Other services | 2018 |
| Onek Records Ltd | Information & communication | 2026 |
| Onelife Adventure Limited | Administrative & support | 2006 |
| Onetec Rentals Ltd | Real estate | 2025 |
| Onethreefour Ltd | Wholesale & retail | 2020 |
| Onewizard Ltd | Information & communication | 2011 |
| Onlyelite Limited | Professional & scientific | 2025 |
| ONT Sofa Ltd | Information & communication | 2011 |
| Onyx Building Ltd | Construction | 2023 |
| Onyx Consultancy Ltd | Professional & scientific | 2023 |
| Onyx ICT Limited | Information & communication | 2012 |
| Onyx Solar Ltd | Construction | 2023 |
| Opal Bear Holdings Ltd | Real estate | 2025 |
| Opal Lodge Holidays Limited | Hospitality | 2022 |
| Opal Projects Limited | Hospitality | 1996 |
| Open Plan & Alfresco Living Limited | Construction | 2016 |
| Open Velocity Limited | Professional & scientific | 2022 |
| Openvoice Limited | Information & communication | 2011 |
| Opti-Twin Ltd | Professional & scientific | 2022 |
| Optibiotix Health PLC | Professional & scientific | 2006 |
| Optibiotix Limited | Professional & scientific | 2012 |
| Optimal Bookkeeping & Accountancy Services Ltd | Administrative & support | 2022 |
| Optimal Financial Planning Ltd | Professional & scientific | 2023 |
| Optimised AV Ltd | Information & communication | 2022 |
| Optimum Bikes Ltd | Wholesale & retail | 2013 |
| Optimum Commercial Projects & Interiors Limited | Professional & scientific | 2013 |
| Optimum HIP and Knee Clinic Ltd | Health & social care | 2024 |
| Optimum Mechanical & Electrical Limited | Manufacturing | 2013 |
| Optimum Vans Ltd | Wholesale & retail | 2025 |
| OPTIMUS4IT Ltd | Information & communication | 2017 |
| Option 3 QS Limited | Administrative & support | 2016 |
| Option a Supply Chain Solutions 2019 Ltd | Professional & scientific | 2019 |
| Optisoft Ltd | Information & communication | 1991 |
| Optiware Limited | Information & communication | 2002 |
| Optix (Group) Limited | Information & communication | 2014 |
| Optix Outsource Ltd | Administrative & support | 2025 |
| Optix Software Ltd | Information & communication | 2002 |
| Opus Recruitment Ltd | Administrative & support | 2018 |
| Orange APE Limited | Information & communication | 2013 |
| Orange Sun Limited | Wholesale & retail | 2020 |
| ORB Implants Limited | Health & social care | 2022 |
| Orbis Property Services Ltd | Real estate | 2020 |
| ORBIT4RAIL Ltd | Information & communication | 2023 |
| Orbital Finance Ltd | Financial services | 2022 |
| Orbitor Technologies Ltd | Information & communication | 2026 |
| Orchard Court (York) Management Company Limited | Households | 2010 |
| Orchard Croft (Diss) Residents Management Company Limited | Households | 2018 |
| Orchard Grove (Coxheath) Residents Management Company Ltd | Households | 2018 |
| Orchard Manor (Cheddington) Residents Management Company Limited | Households | 2019 |
| Orchard Meadows (Iwade) Residents Management Company Limited | Households | 2022 |
| Orchard Trees Childcare Ltd | Education | 2016 |
| Orchard Trees Day Nursery Limited | Health & social care | 2008 |
| Orchid Dental Projects Ltd | Construction | 2025 |
| Orchid Vegan Ltd | Wholesale & retail | 2024 |
| Ordc Limited | Health & social care | 2025 |
| Ordinary 2 Extraordinary | Arts & recreation | 2006 |
| Ordinary Athlete Limited | Wholesale & retail | 2020 |
| Ordinary Better Ltd | Wholesale & retail | 2020 |
| Organic-Essentials Limited | Wholesale & retail | 2020 |
| Orgella Professional Ltd | Other services | 2003 |
| Oriental Contract Services Limited | Administrative & support | 2022 |
| Origamia Ltd | Professional & scientific | 2014 |
| Origin Group Finance Limited | Financial services | 2013 |
| Origin Group Holdco Limited | Financial services | 2013 |
| Original York Ltd | Information & communication | 2005 |
| Orillo Limited | Wholesale & retail | 2013 |
| Orini Limited | Professional & scientific | 2014 |
| Orion Home Improvements Limited | Construction | 2024 |
| Orion Post Ltd | Information & communication | 2025 |
| Oris Construction Ltd | Construction | 2002 |
| Orlando Consultants Limited | Professional & scientific | 2020 |
| Orlando's Vintage Ltd | Wholesale & retail | 2021 |
| Ormesby Interiors Limited | Construction | 2018 |
| Ornhams Holdings Ltd | Real estate | 2018 |
| Orthellius Ltd | Extraterritorial | 2016 |
| Orthodontic.Works Ltd | Health & social care | 2025 |
| Orwak Easi UK Ltd | Wholesale & retail | 1999 |
| OSB81 Limited | Health & social care | 2024 |
| Osborne House Edinburgh Limited | Construction | 2017 |
| Osbornes of York Limited | Hospitality | 2016 |
| Oscar Ionisation Ltd | Hospitality | 2021 |
| Oscars Bar (Pocklington) Limited | Hospitality | 2010 |
| Oscillate Studio Limited | Professional & scientific | 2025 |
| Osher Ltd | Real estate | 2021 |
| Osmogenesis Holdings Ltd | Financial services | 2025 |
| Osmogenesis Media Ltd | Information & communication | 2025 |
| OSO Ventures I LLP | | 2025 |
| Osprey L & S Systems Ltd | Administrative & support | 2024 |
| Ossie Studio Ltd | Arts & recreation | 2023 |
| Ostc Land Ltd | Extraterritorial | 2016 |
| OSW Projects Ltd | Construction | 2012 |
| OT4ME Ltd | Health & social care | 2020 |
| Otani Limited | Professional & scientific | 2014 |
| Otis Hedge Ltd | Real estate | 2024 |
| Otisfam Ltd | Hospitality | 2023 |
| Otterham Park (Rainham) Residents Management Company Limited | Households | 2020 |
| Otterweave Games Studio Ltd | Information & communication | 2024 |
| Otterwood Developments Ltd | Real estate | 2024 |
| Otterwood Home Ltd | Wholesale & retail | 2025 |
| Ouch Academy Ltd | Health & social care | 2024 |