| O&B Group Ltd | Construction | 2022 |
| O&G Cleaning Services Limited | Administrative & support | 2024 |
| O&S Enterprise Ltd | Other services | 2014 |
| O'Shea, Construction Solutions Limited | Manufacturing | 2019 |
| O.C.L. Drylining Limited | Construction | 2008 |
| O.C.L. Facades Limited | Construction | 2003 |
| O.C.L. Holdings Limited | Financial services | 2021 |
| O.C.L. Limited | Extraterritorial | 2022 |
| O.C.L. Management Services Limited | Professional & scientific | 2003 |
| O.C.L. Reclad Limited | Construction | 2021 |
| OA Property Solutions Ltd | Real estate | 2021 |
| Oabl Ltd | Administrative & support | 2025 |
| Oak Heath (Brentwood) Management Company Limited | Households | 1983 |
| Oakbrook Capital (I) PLC | Financial services | 2015 |
| Oakclay UKS Limited | Construction | 2025 |
| Oakenson Ltd | Construction | 2024 |
| Oakhills Care Services Ltd | Health & social care | 2019 |
| Oakhouse Living Spaces Ltd | Real estate | 2018 |
| Oaklands Day Project Ltd | Health & social care | 2026 |
| Oakley ERP Solutions Ltd | Information & communication | 2019 |
| Oakley Global Business Solutions Ltd | Information & communication | 1999 |
| Oakley Hanger Consultancy Limited | Arts & recreation | 2026 |
| Oakley London Limited | Construction | 2016 |
| Oakmore MGT Ltd | Professional & scientific | 2026 |
| Oaks Access & Maintenance Ltd | Construction | 2026 |
| Oasis Group Training Limited | Arts & recreation | 2025 |
| Oasis Health & Fitness Club Limited | Administrative & support | 2006 |
| Oasis Housing Management Ltd | Hospitality | 2021 |
| Oasis Ladies Limited | Health & social care | 2012 |
| Oasis Meadows Assisted Living Ltd | Administrative & support | 2020 |
| Oasis Wellness Limited | Administrative & support | 2025 |
| Oasis Women Limited | Health & social care | 2018 |
| Oatham Clark & Co Ltd | Financial services | 2016 |
| OB Electrics Ltd | Construction | 2024 |
| OB Group Services Limited | Construction | 2022 |
| OB Unique Ltd | Construction | 2020 |
| OBE to DUN Afrohub Limited | Wholesale & retail | 2022 |
| Obsidian Achernar Ltd | Administrative & support | 2017 |
| OBUKO65 Limited | Health & social care | 2022 |
| Ocean Rider Ltd | Wholesale & retail | 2026 |
| Oceans Transport Ltd | Transport & logistics | 2020 |
| Ocius Limited | Administrative & support | 2004 |
| Ocobrown Health Limited | Professional & scientific | 2026 |
| Octagon Data Limited | Information & communication | 2009 |
| Octav Construct Ltd | Construction | 2021 |
| Octav Holding Limited | Real estate | 2025 |
| Octavelite Builders Group Ltd | Construction | 2024 |
| Octavian Marin Limited | Construction | 2020 |
| Octavius Electrical Solutions Limited | Construction | 2021 |
| Odatam Ltd | Professional & scientific | 2020 |
| Odessa BG Group Limited | Construction | 2019 |
| Odua Foods Limited | Wholesale & retail | 2020 |
| OES Electrics Limited | Construction | 2021 |
| Ofemzy Global Limited | Real estate | 2025 |
| OFF Grid Group Ltd | Construction | 2023 |
| OFF the PEG Design Limited | Professional & scientific | 2014 |
| Office Boss Limited | Administrative & support | 2023 |
| Office Furniture Group Ltd | Construction | 2020 |
| Office Group Installations Ltd | Construction | 2023 |
| Office Needs Group Limited | Professional & scientific | 2016 |
| Office Needs Limited | Wholesale & retail | 1997 |
| Offsite Consulting Limited | Professional & scientific | 2014 |
| OG Performance Ltd | Wholesale & retail | 2025 |
| OG Prestige Ltd | Wholesale & retail | 2024 |
| OH My! Productions Limited | Information & communication | 1999 |
| OJK Drylining Company Limited | Construction | 2025 |
| OJO Holdings Ltd | Financial services | 2025 |
| OKE Development Ltd | Construction | 2022 |
| Okwesco Ventures Limited | Professional & scientific | 2012 |
| Okwudili Limited | Health & social care | 2013 |
| Oladunni Ltd | Health & social care | 2022 |
| Olaniyan Tech Service Limited | Information & communication | 2025 |
| Olatunboss Ltd | Administrative & support | 2025 |
| OLC Trans Ltd | Transport & logistics | 2016 |
| Old and New for YOU Ltd | Wholesale & retail | 2023 |
| Old Barbers Limited | Other services | 2021 |
| Old Farm Court (Billericay) Limited | Real estate | 1988 |
| Old LEE Aggregates Limited | Transport & logistics | 2025 |
| Oldtown: Love & Sonder Limited | Wholesale & retail | 2022 |
| Olec Services Ltd. | Construction | 2018 |
| Olfino Ltd | Wholesale & retail | 2020 |
| Olice Limited | Administrative & support | 2021 |
| Olikud Ltd | Real estate | 2025 |
| Olimar Construct Ltd | Construction | 2026 |
| Olive Banquet Ltd | Hospitality | 2020 |
| Olive Data Consulting Ltd | Information & communication | 2023 |
| Olive Shoots Limited | Information & communication | 2006 |
| Oliver Price Limited | Manufacturing | 2003 |
| Olla Property Services Ltd | Administrative & support | 2026 |
| Ollie Cleaning Services Ltd | Administrative & support | 2013 |
| Olsem Securities Limited | Wholesale & retail | 2025 |
| Oltiana Thoma Limited | Health & social care | 2019 |
| OLU International UK Limited | Construction | 2015 |
| Olubicks & KD Services Ltd | Transport & logistics | 2019 |
| Olympus Electrical & Building Services Ltd | Construction | 2017 |
| Omadak Limited | Wholesale & retail | 2022 |
| Omar Hearing Services (UK) Limited | Administrative & support | 2013 |
| Omecan UK Limited | Other services | 2015 |
| Omgifting Ltd. | Other services | 2018 |
| Omis Engineering Limited | Extraterritorial | 2016 |