| OG Catering Ltd | Hospitality | 2023 |
| Ogao Limited | Information & communication | 2020 |
| Ogfl Properties Ltd | Real estate | 2025 |
| Oglesby-Wass Solutions Limited | Construction | 2014 |
| Ogodo Services Ltd | Administrative & support | 2017 |
| Ogsten Ltd | Construction | 2022 |
| OGV Ltd | Real estate | 2023 |
| OH Hello Darling Ltd | Information & communication | 2025 |
| OH My Marketing Ltd | Professional & scientific | 2023 |
| OH SO Delicious Ltd | Hospitality | 2020 |
| OHB Music Ltd | Arts & recreation | 2026 |
| OHL Ltd | Administrative & support | 1984 |
| OHR Community Healthcare Ltd | Administrative & support | 2022 |
| OHR Services Ltd | Public administration | 2020 |
| OIL Heating Consultancy Services Ltd. | Professional & scientific | 2014 |
| Oils From Nature Limited | Extraterritorial | 2022 |
| Oj&Aj Ltd | Transport & logistics | 2023 |
| Ok&Ko Ltd | Transport & logistics | 2016 |
| OKA Trans Ltd. | Transport & logistics | 2008 |
| Okeh Properties Limited | Real estate | 2000 |
| Okoma Apartments Ltd | Real estate | 2020 |
| OLA Consultancy Ltd | Financial services | 2014 |
| Olavee Autocare & Sales Limited | Wholesale & retail | 2025 |
| Old Burlington Street General Partner Limited | Construction | 2009 |
| Old Co-Op Annesley Ltd | Real estate | 2023 |
| Old Hall Services Management Company (Woodford) Limited | Households | 2024 |
| Old MAN River 2 Ltd | Real estate | 2024 |
| Old MAN River Ltd | Real estate | 2024 |
| Old Paper Shop Holdings Limited | Financial services | 2017 |
| Old Station Quarter Limited | Hospitality | 2025 |
| Old Trinity Management Co Ltd | Households | 2025 |
| Olfactory Services Limited | Wholesale & retail | 2025 |
| Olha Delivery Ltd | Transport & logistics | 2017 |
| Olimax Property Developers Ltd | Real estate | 2021 |
| Olimpia&Clean Ltd | Administrative & support | 2025 |
| Olisadaniel Medical Consultancy Limited | Health & social care | 2010 |
| Olite Ltd | Wholesale & retail | 2024 |
| Olive and IVY Ltd | Hospitality | 2026 |
| Olive Care Solutions Ltd | Health & social care | 2022 |
| Olive Club Ltd | Hospitality | 2026 |
| Olive Grove Properties Ltd | Real estate | 2022 |
| Olive Print and Design Ltd | Administrative & support | 2019 |
| Olive Tree Centres Limited | Health & social care | 2025 |
| Olivella Ltd | Other services | 2020 |
| Oliver Lettings Gibraltar Limited | Real estate | 2025 |
| Oliver Lettings Ltd | Real estate | 2022 |
| Oliver Solutions Group Limited | Professional & scientific | 2024 |
| Oliver's Kitchen Products Limited | Manufacturing | 2017 |
| Olives & Grains Ltd | Wholesale & retail | 2025 |
| Olivia Rose Wilkes Ltd | Wholesale & retail | 2026 |
| Olivier Tyres Ltd | Manufacturing | 2024 |
| Olmart Limited | Health & social care | 2011 |
| Olympia Building & Carpentry Ltd | Construction | 2016 |
| Olympus Automation Limited | Manufacturing | 1997 |
| Olympus Consultancy Limited | Professional & scientific | 2017 |
| Olympus Investments Limited | Real estate | 2017 |
| Olympus Property Management Limited | Real estate | 2017 |
| OM Car Wash Ltd | Other services | 2024 |
| OM Namah Shivaya Properties Ltd | Real estate | 2020 |
| OM Optical Limited | Wholesale & retail | 2018 |
| OM Trans Ltd | Transport & logistics | 2025 |
| OMA Properties Cambs Ltd | Real estate | 2022 |
| Oma-Trans Limited | Transport & logistics | 2016 |
| Omega City Group Limited | Financial services | 2022 |
| Omega Sinto Foundry Machinery Limited | Manufacturing | 1996 |
| Omega Staff Recruitment Ltd | Administrative & support | 2024 |
| Omegadesign Galati Limited | Wholesale & retail | 2025 |
| OMM Print Group Ltd | Manufacturing | 2026 |
| Omni Gaming Limited | Information & communication | 2023 |
| Omnia Perinatal Systems (OPS) Ltd | Information & communication | 2026 |
| Omnihusk Ltd | Administrative & support | 2025 |
| Omnivent UK Limited | Manufacturing | 2017 |
| Omogaide Enterprises Ltd | Wholesale & retail | 2025 |
| OMR Electrical Ltd | Construction | 2025 |
| Omshree Ltd | Real estate | 2024 |
| ON Point Building and Property Solutions Ltd | Construction | 2026 |
| ON Technologies Limited | Information & communication | 2025 |
| ON the Go Logistics Ltd | Transport & logistics | 2025 |
| ON the Level Carpentry Services Limited | Construction | 2012 |
| On-Call London Ltd | Manufacturing | 2022 |
| ON2COURT Limited | Arts & recreation | 2025 |
| ON2COURT Pickleball Limited | Arts & recreation | 2026 |
| Onco-It Consultancy Ltd | Information & communication | 2025 |
| Oncology Expert Ltd | Health & social care | 2014 |
| Oncology Property Investment Ltd | Real estate | 2024 |
| One 17 Autos Ltd | Wholesale & retail | 2023 |
| One 9 Nine Ltd | Wholesale & retail | 2023 |
| One Acre West Street LLP | | 2020 |
| One Harmony Ltd | Health & social care | 2025 |
| One More Brick Ltd | Wholesale & retail | 2021 |
| One More Step Limited | Professional & scientific | 2026 |
| One OR Two Properties Ltd | Real estate | 2017 |
| One Printer Ltd | Manufacturing | 2019 |
| One Square Meter Limited | Information & communication | 2023 |
| One Stop Auto Group Ltd | Wholesale & retail | 2026 |
| One Stop Downham Ltd | Wholesale & retail | 2020 |
| One Stop Nova Ltd | Wholesale & retail | 2025 |
| One Stop Welding Supplies Limited | Extraterritorial | 2016 |
| One Twenty-One Ltd | Other services | 2025 |
| One Window Ltd | Wholesale & retail | 2026 |