| Oak Letco Limited | Real estate | 2025 |
| Oakbank Care Home Limited | Health & social care | 2021 |
| Oakbay Ltd | Professional & scientific | 2000 |
| Oakesway Estates (2) Limited | Extraterritorial | 2024 |
| Oakesway Estates Limited | Extraterritorial | 2024 |
| Oakmount Property Services Limited | Construction | 2020 |
| Oakstar Investments Limited | Real estate | 2025 |
| Oakstream Investments Limited | Real estate | 2022 |
| Oakwell Prime Estates Ltd | Real estate | 2025 |
| Oakwood Engineering Solutions Limited | Construction | 2011 |
| Oasis Engineering Solutions Ltd | Manufacturing | 2013 |
| Oasis M7 Limited | Health & social care | 2025 |
| Oasis Spaces Ltd | Construction | 2025 |
| OB Telecom Limited | Information & communication | 2021 |
| OBI Health and Allied Services Limited | Health & social care | 2023 |
| Oblitus International Ltd | Other services | 2025 |
| OBZ Holdings Limited | Financial services | 2026 |
| OC Home Stays Ltd | Real estate | 2021 |
| OC Property Developments Ltd | Real estate | 2021 |
| OC Property Ltd | Real estate | 2019 |
| OC SA Management Ltd | Households | 2020 |
| Ocean Forest UK Ltd | Financial services | 2017 |
| Ocean-T Business Limited | Transport & logistics | 2024 |
| Oceanic Global Transactions Limited | Other services | 2023 |
| Oceanmarble Limited | Wholesale & retail | 2023 |
| Oceanprintsuk Ltd | Manufacturing | 2024 |
| Oceanpuilse Ltd | Real estate | 2025 |
| Oceanpulse Ltd | Construction | 2025 |
| Oceanpulse OY Ltd | Wholesale & retail | 2025 |
| Oceansite Limited | Real estate | 2013 |
| Oceanview Property Limited | Professional & scientific | 2014 |
| Octoform Limited | Professional & scientific | 2026 |
| Oculaw Limited | Administrative & support | 2007 |
| Odavies Property Ltd | Real estate | 2023 |
| Oddity Acting LLP | | 2026 |
| Odeko Estates Limited | Real estate | 2021 |
| Odsy Limited | Arts & recreation | 2023 |
| OEM Appliances Limited | Wholesale & retail | 2012 |
| Ofadamanchester Ltd | Manufacturing | 2023 |
| OFF U Go! Limited | Administrative & support | 2022 |
| Offerupbid Limited | Wholesale & retail | 2025 |
| Officina Anglica Ltd | Information & communication | 2025 |
| Offline Solutions Ltd | Wholesale & retail | 2017 |
| OG Innovation Pro Limited | Manufacturing | 2026 |
| OH! Video Limited | Information & communication | 2015 |
| OHM Capital (UK) Ltd | Real estate | 2025 |
| OHM Professional Services Ltd | Professional & scientific | 2026 |
| Oholei Yosef Yitzchok Lubavitch Schools | Other services | 2015 |
| OIC3 Auto Services Limited | Wholesale & retail | 2025 |
| OIL Sites Ltd | Extraterritorial | 2012 |
| OJ Global Resources Limited | Wholesale & retail | 2013 |
| OJ Security Solutions & Services Limited | Administrative & support | 2024 |
| OJ Solutions Services Limited | Information & communication | 2017 |
| Okad Ltd | Wholesale & retail | 2025 |
| Okusanya Ltd | Real estate | 2024 |
| OLA Energy Ethiopia Limited | Wholesale & retail | 1929 |
| Ola's Salford Ltd | Wholesale & retail | 2019 |
| Olala Crepes Ltd | Hospitality | 2024 |
| Old Bowling Green Management Company (Ashbourne) Limited | Households | 2024 |
| Old Hall Assets Limited | Real estate | 2024 |
| Old Hall Assets SPV1 Limited | Real estate | 2026 |
| Old Trafford Consulting Limited | Professional & scientific | 2011 |
| Oldbury Hazel Limited | Professional & scientific | 2025 |
| Olde Worlde Creations Ltd | Wholesale & retail | 2025 |
| Oldeka Ltd | Health & social care | 2017 |
| Oldham Financial Consultancy Services Limited | Professional & scientific | 2017 |
| Oliexpres. Ltd | Transport & logistics | 2018 |
| Oliey Ltd | Wholesale & retail | 2025 |
| Oliva Takeaway Ltd | Hospitality | 2026 |
| Olivepond Investments Limited | Real estate | 2022 |
| Oliver's Barbers Manchester Limited | Other services | 2019 |
| Olivian Vans & Cars Sales Ltd | Wholesale & retail | 2022 |
| Olivierdun Ltd | Wholesale & retail | 2025 |
| Olivion Capital Ltd | Professional & scientific | 2025 |
| Olivion E-Commerce Group Ltd | Administrative & support | 2025 |
| Olivion Hair Ltd | Wholesale & retail | 2025 |
| Ollyville Ltd | Extraterritorial | 2008 |
| Olmewood Limited | Professional & scientific | 1981 |
| Olora Fields UK Limited | Real estate | 2024 |
| Olufinn Limited | Wholesale & retail | 2025 |
| Oluvic (UK) Ltd | Health & social care | 2013 |
| Omega Line Constructions Ltd | Construction | 2025 |
| Omega Payroll Services Ltd | Administrative & support | 2015 |
| Omega Resources (UK) Ltd. | Professional & scientific | 2024 |
| OMI Manchester Limited | Financial services | 2019 |
| Omid Dentcare Ltd | Health & social care | 2012 |
| Omihon Limited | Information & communication | 2021 |
| Omio Group Ltd | Wholesale & retail | 2026 |
| Omnibridge Education Solutions Ltd | Education | 2026 |
| Omnibridge Language Solutions Ltd | Professional & scientific | 2026 |
| Omnicare Recruitment Limited | Administrative & support | 2019 |
| Omnigrowth Ltd | Information & communication | 2020 |
| Omniversity Group Limited | Education | 2023 |
| Omniversity Limited | Professional & scientific | 2023 |
| Omnniah Ltd | Wholesale & retail | 2025 |
| Omo-Chiefcare Limited | Health & social care | 2023 |
| ON Height Ltd | Wholesale & retail | 2022 |
| ON Space Developments Limited | Construction | 2022 |
| ON Track Admissions Ltd | Education | 2026 |
| Onas Drive Ltd | Education | 2019 |