| Offshore RSS Limited | Wholesale & retail | 2023 |
| OH Shala Limited | Wholesale & retail | 2025 |
| Ohaana Halal Chinese Ltd | Hospitality | 2024 |
| OI Garments Ltd | Wholesale & retail | 2026 |
| OIL Water Stop Limited | Manufacturing | 2024 |
| Okabe Automotive Ltd | Wholesale & retail | 2025 |
| Oktein Holdings Limited | Information & communication | 2019 |
| Oktein Interactive Ltd | Information & communication | 2018 |
| Oktein Technology Limited | Information & communication | 2019 |
| Old Coach House Inn Limited | Hospitality | 2024 |
| Old Forge Court (Loudwater) Limited | Extraterritorial | 1999 |
| Old Sun Management Ltd | Households | 2014 |
| Old Vicarage Construction Ltd | Construction | 2012 |
| Old Vikings Ltd | Other services | 2024 |
| Olive Tree High Wycombe Limited | Hospitality | 2023 |
| Oliveco Limited | Wholesale & retail | 2016 |
| Oliver House Limited | Construction | 2015 |
| Oliver Johnstone Software Ltd | Information & communication | 2018 |
| Oliver Stone Ltd | Manufacturing | 2019 |
| Olixan Group Limited | Transport & logistics | 2025 |
| OM Bespoke Limited | Manufacturing | 2021 |
| OM Manufacturing Limited | Manufacturing | 2022 |
| OM SVK UK Ltd | Wholesale & retail | 2023 |
| OM Transport Services Ltd | Transport & logistics | 2024 |
| OM2 Ltd | Real estate | 2021 |
| Omaghcom Limited | Health & social care | 2002 |
| Omar Farooq Estates Ltd | Real estate | 2023 |
| Omega Energy Insights Ltd | Professional & scientific | 2024 |
| Omnav Limited | Hospitality | 2019 |
| Omni Decorating Ltd | Construction | 2022 |
| Omni Plastering Limited | Construction | 2016 |
| Omni Wellbeing Ltd | Information & communication | 2022 |
| Omnicrest Construction Limited | Construction | 2016 |
| Omnidigit Solutions Limited | Professional & scientific | 2003 |
| Omnistar Ltd | Real estate | 2026 |
| Omnium Associates Limited | Professional & scientific | 2022 |
| ON Guard Dental Laboratory Limited | Professional & scientific | 2017 |
| ON Point Grill Ltd | Hospitality | 2024 |
| ON TAP Solutions Ltd | Construction | 2024 |
| ON the Mark Ltd | Other services | 2026 |
| ON Your Side Financial Solutions Limited | Professional & scientific | 2020 |
| Onbio Ltd | Administrative & support | 2014 |
| One Away by MMM Ltd | Hospitality | 2022 |
| One CAN Trust Ltd | Health & social care | 2012 |
| One Giant Step Limited | Professional & scientific | 2022 |
| One Ship Solutions Ltd | Transport & logistics | 2025 |
| One Springfield Ltd | Real estate | 2024 |
| One Stop Phone & Vape Store Ltd | Other services | 2023 |
| One World Estate Ltd | Real estate | 2023 |
| ONE2CONSULT Ltd | Professional & scientific | 2023 |
| Onefocus IT Ltd | Information & communication | 2021 |
| Oneho Energy Solutions UK Ltd | Construction | 2026 |
| Onesiphorus Ltd | Information & communication | 2020 |
| Onesource Services (UK) Limited | Professional & scientific | 2025 |
| Oneway Instructor and Driver Training Ltd | Education | 2008 |
| Onhol Limited | Financial services | 2018 |
| Onix Ltd | Wholesale & retail | 2024 |
| Online Quran Tuition Ltd | Education | 2024 |
| Only the Best Collectables Ltd | Wholesale & retail | 2025 |
| Only Trade Cars Limited | Wholesale & retail | 2021 |
| Ontemp Ltd | Administrative & support | 2025 |
| Onthego Taxis Ltd | Transport & logistics | 2019 |
| Onward EV Ltd | Administrative & support | 2023 |
| Onyx Brokers Limited | Financial services | 2013 |
| Onyx Motor Sales Ltd | Wholesale & retail | 2021 |
| Onyx Tours Limited | Administrative & support | 2009 |
| Opal Design Studio Ltd | Professional & scientific | 2025 |
| Opal Events Ltd | Hospitality | 2024 |
| Opar Holdings Ltd | Manufacturing | 2019 |
| Opatha Consultants Limited | Administrative & support | 2016 |
| Open Computing Alliance International Limited | Information & communication | 2020 |
| Open House Micklefield CIC | Hospitality | 2015 |
| Openeq Ltd | Information & communication | 2024 |
| Opnplay Limited | Information & communication | 2024 |
| Oppeneo Ltd | Professional & scientific | 2026 |
| Opsmonks Ltd | Information & communication | 2017 |
| Optic Visionaries Ltd | Health & social care | 2022 |
| Optical Disc Services Ltd | Manufacturing | 2024 |
| Optima Contracting Limited | Construction | 1985 |
| Optima Glass Installations Limited | Professional & scientific | 2011 |
| Optima Homecare Group Limited | Health & social care | 2014 |
| Optima Installations Limited | Construction | 1997 |
| Optima Investments Limited | Financial services | 2008 |
| Optima Partitioning Systems Limited | Professional & scientific | 1986 |
| Optima Products Limited | Manufacturing | 1971 |
| Optimal Audio Group Limited | Manufacturing | 2018 |
| Optimise Health Solutions Limited | Health & social care | 2022 |
| Optimity AI Tech (UK) Ltd | Information & communication | 2026 |
| Optimum Packing Ltd | Transport & logistics | 2018 |
| Optimus Elite Ltd | Construction | 2014 |
| Optinett (UK) Limited | Manufacturing | 1989 |
| Optionz Hair & Beauty Ltd | Other services | 2022 |
| Optiva Consultancy Limited | Professional & scientific | 2025 |
| Opto Live Limited | Information & communication | 2018 |
| Optotutor Limited | Health & social care | 2019 |
| Opulart Ltd | Professional & scientific | 2023 |
| Opulence Bathrooms and Heating Systems Ltd | Construction | 2025 |
| Oqwado Ltd | Information & communication | 2020 |
| Oralvet Referrals Limited | Professional & scientific | 2025 |
| Orange People Limited | Construction | 2015 |