| Ogobia Consultancy Services Limited | Financial services | 2019 |
| Ogoejor Health Care Limited | Health & social care | 2025 |
| Ogoejor Rental Services Limited | Real estate | 2019 |
| Ogoejor Services Limited | Real estate | 2018 |
| Ogunsuyi Ltd | Financial services | 2018 |
| OGV Rail Ltd | Construction | 2016 |
| Ohanz Limited | Health & social care | 2022 |
| Oheneba Ltd | Health & social care | 2021 |
| Ohi-Soft Limited | Information & communication | 2024 |
| OK to Colour Limited | Manufacturing | 2016 |
| Okafor and Sons Limited | Health & social care | 2026 |
| Okechukwu Okoro Locum GP Ltd | Health & social care | 2026 |
| OKO Welfare Association | Other services | 2012 |
| Ol'Ajax Costrend Limited | Professional & scientific | 2018 |
| OLA Ventures Ltd | Wholesale & retail | 2022 |
| OLA Yusuf Ltd | Professional & scientific | 2024 |
| Olaas Treats Limited | Wholesale & retail | 2025 |
| Olad Medics Limited | Health & social care | 2020 |
| Olana Care Ltd | Health & social care | 2026 |
| Olaolu Limited | Information & communication | 2009 |
| Olaray Consulting UK Ltd | Administrative & support | 2019 |
| Olat Group Limited | Wholesale & retail | 2025 |
| Old Harlow OFF Licence Ltd | Wholesale & retail | 2025 |
| Old Jack Company Limited | Health & social care | 2022 |
| Oldco Transition Limited | Professional & scientific | 2021 |
| Oldee Ltd | Real estate | 2024 |
| Oleksy Builders Limited | Construction | 2015 |
| Olive Barn Nursery Ltd | Health & social care | 2022 |
| Olive General Practice Limited | Health & social care | 2024 |
| Olive Grange Healthcare Ltd | Health & social care | 2023 |
| Olivenite Limited | Wholesale & retail | 2021 |
| Oliver&Parkerconstruction Ltd | Construction | 2023 |
| Olivia Burnham Properties Ltd | Real estate | 2024 |
| Olivier Kugler Ltd | Other services | 2010 |
| Olivine Limited | Real estate | 2021 |
| Oloke Ltd | Health & social care | 2022 |
| Olotu Primary Care Ltd | Health & social care | 2026 |
| Olowo Couture Ltd | Wholesale & retail | 2023 |
| Olowo Medical Ventures Limited | Health & social care | 2024 |
| Olsam Limited | Wholesale & retail | 2014 |
| Oluwakemib Properties Limited | Real estate | 2018 |
| Oluwatobi Odanye Ltd | Health & social care | 2025 |
| Olympos Properties Ltd | Real estate | 2021 |
| Olza Ltd | Real estate | 2023 |
| OM Electrical Installations Ltd | Construction | 2022 |
| Omaxco Ltd | Wholesale & retail | 2023 |
| Omega Prestige Ltd | Real estate | 2021 |
| OMH Bespoke Builds Ltd | Construction | 2026 |
| Omnifawz Ltd | Information & communication | 2025 |
| Omnipeak Limited | Real estate | 2023 |
| Omnitel Consultancy Limited | Professional & scientific | 2020 |
| Omniya Properties Ltd | Construction | 2023 |
| Omoladeowo Ltd | Health & social care | 2022 |
| Omonu Ltd | Transport & logistics | 2026 |
| ON Point Security Services Limited | Administrative & support | 2022 |
| On-Track Property Services Ltd | Education | 2021 |
| Onaji Limited | Wholesale & retail | 2013 |
| Onas Consult Ltd | Health & social care | 2020 |
| Once & Again Limited | Wholesale & retail | 2026 |
| One and Only Dental Limited | Health & social care | 2020 |
| One Leaf Limited | Information & communication | 2009 |
| One Vision Care Ltd | Health & social care | 2025 |
| Onemore Pro Ltd | Construction | 2018 |
| Onesixty RTM Company Limited | Households | 2010 |
| Oneunique Chauffeuring & Rental Limited | Transport & logistics | 2025 |
| Onewig Onesmile Limited | Health & social care | 2019 |
| Onifade Consulting Services Ltd | Health & social care | 2022 |
| Oniga Building Ltd | Construction | 2014 |
| Online Buddy Ltd | Professional & scientific | 2018 |
| Online Reptile Shop Limited | Wholesale & retail | 2015 |
| Onoro Medical Limited | Health & social care | 2026 |
| Onosetale’S Company Ltd | Health & social care | 2024 |
| Onusfocushomes Ltd | Real estate | 2022 |
| Onusfocustech Ltd | Information & communication | 2017 |
| Onyx Clinix Ltd | Health & social care | 2025 |
| Oops a Daisy UK Ltd | Wholesale & retail | 2020 |
| Opal Technology Ltd | Professional & scientific | 2014 |
| Open Systems Consulting Ltd | Information & communication | 2018 |
| OPI Limited | Construction | 2018 |
| Opic Limited | Real estate | 2025 |
| Optiimum Tech Ltd | Information & communication | 2025 |
| Optima Training & Consultancy Limited | Administrative & support | 2019 |
| Optimal Business Finance Ltd | Financial services | 2023 |
| Optimal Heat Ltd | Construction | 2024 |
| Optimum Care Services Limited | Health & social care | 2023 |
| Options Electricity Services Limited | Construction | 2011 |
| Options Energy EOT Limited | Financial services | 2024 |
| Options Energy Group Limited | Financial services | 2014 |
| Options Energy Services Limited | Construction | 2011 |
| Options Energy Support LLP | | 2014 |
| Options Telecom Services Limited | Construction | 2011 |
| Options Water Services Limited | Construction | 2011 |
| Options World UK Ltd | Manufacturing | 2017 |
| Optix (UK) Ltd | Professional & scientific | 2010 |
| Opus Line Limited | Professional & scientific | 2026 |
| ORA Collective Ltd | Professional & scientific | 2025 |
| Orangetrade Limited | Wholesale & retail | 2013 |
| Orca Roofing Ltd | Construction | 2025 |
| Orchard 1967 Limited | Extraterritorial | 2014 |
| Orchard Court (Hitchin) Management Company Limited | Households | 2020 |