| Omni Management Group Ltd | Real estate | 2025 |
| Omni Mortgages Ltd | Financial services | 2025 |
| Omnia Care Leicester Limited | Real estate | 2021 |
| Omnist Ltd | Information & communication | 2021 |
| OMTECH1 Limited | Wholesale & retail | 2016 |
| Onam Holdings Limited | Real estate | 2020 |
| One Mineral Limited | Wholesale & retail | 2024 |
| One Morshead Road Management Company Limited | Real estate | 1994 |
| One N1 Residents Limited | Real estate | 2005 |
| One Nature Holdings Ltd | Financial services | 2021 |
| One Nature Retreats Ltd | Extraterritorial | 2021 |
| One Stop Consulting Ltd | Professional & scientific | 2023 |
| One Stop Restaurant Ltd | Hospitality | 2025 |
| Oneclick Media Services Ltd | Information & communication | 2003 |
| Onesource Display Services Ltd | Manufacturing | 2023 |
| ONH Ltd | Construction | 2025 |
| Only London Ltd | Real estate | 2016 |
| Only Slightly Ltd | Arts & recreation | 2013 |
| Onpoint Property Portfolio Limited | Real estate | 2024 |
| ONPOINTON9TUTORS Ltd | Education | 2022 |
| Onyx Investment Properties Limited | Real estate | 2017 |
| Opal Design & Build Ltd | Construction | 2024 |
| Opal GEM Ltd | Real estate | 2021 |
| Opco Advisory UK Limited | Financial services | 2026 |
| Openxccess Limited | Information & communication | 2017 |
| Opes Management Limited | Real estate | 2025 |
| Optima Medical Group Ltd | Health & social care | 2026 |
| Optima Retail Holdings Ltd | Wholesale & retail | 2025 |
| Optima Retrofit Solutions Ltd | Construction | 2019 |
| Optimand Ltd | Information & communication | 2017 |
| Optimising Advance Green Industries Ltd | Wholesale & retail | 2016 |
| Optimism Limited | Administrative & support | 2014 |
| Optimum Performance Solutions International Ltd | Professional & scientific | 2018 |
| Opus Associates Limited | Wholesale & retail | 2023 |
| Opus Design and Build Limited | Construction | 2023 |
| Opus Studios RTM Ltd | Real estate | 2007 |
| Orange Blossom Limited | Wholesale & retail | 2019 |
| Orangepay Global Ltd | Information & communication | 2025 |
| Orangerock Holdings Ltd | Financial services | 2025 |
| Orbe Design & Media Limited | Arts & recreation | 2012 |
| Orbital Vision Ltd | Health & social care | 2013 |
| Orbiton Investments Limited | Real estate | 1977 |
| Organii Edge Limited | Wholesale & retail | 2025 |
| Origins Properties Limited | Real estate | 2025 |
| Orion House Development Limited | Real estate | 2014 |
| ORM Capital Investments Ltd | Real estate | 2019 |
| ORM Group Ltd | Professional & scientific | 2018 |
| ORM Holdings Limited | Financial services | 2016 |
| ORM Systems UK Ltd. | Wholesale & retail | 2016 |
| Ortaq+ Healthcare Ltd | Wholesale & retail | 2025 |
| Orvaal Limited | Information & communication | 2004 |
| Orxa Grid Ltd | Professional & scientific | 2017 |
| Osadhi Limited | Wholesale & retail | 2024 |
| Oshin Properties Ltd | Households | 2015 |
| Oshwal Trading Limited | Wholesale & retail | 2013 |
| Oslerpharm Limited | Wholesale & retail | 2024 |
| OSM Electrical Limited | Construction | 2019 |
| OTE Pharma UK Limited | Wholesale & retail | 2014 |
| Otifyd Limited | Wholesale & retail | 2022 |
| Ottercroft Limited | Administrative & support | 1984 |
| Ourganic Limited | Hospitality | 2019 |
| Outcompute Ltd | Information & communication | 2019 |
| Outland Developments Ltd | Construction | 2025 |
| Outline Strategy Ltd | Professional & scientific | 2025 |
| Outpost Design Ltd | Professional & scientific | 2011 |
| Ovolo Brands and Marketing Limited | Professional & scientific | 2014 |
| Ovrwlm Ltd | Information & communication | 2024 |
| OW Stanley Properties Ltd | Real estate | 2022 |
| OWL Barn Trustee Limited | Financial services | 2022 |
| Ownet Technologies Ltd | Information & communication | 2019 |
| Oxford 21 Limited | Information & communication | 2022 |
| Oxford Pharma Limited | Other services | 2018 |
| Oxford Zeus Limited | Professional & scientific | 2018 |
| Oxycloud Limited | Wholesale & retail | 2025 |
| Oyster Rentals Ltd | Real estate | 2020 |
| Oyston Wealth Management Limited | Financial services | 2006 |
| Ozata Ltd | Wholesale & retail | 2026 |
| Oznetworks Ltd | Information & communication | 2023 |
| P & D Courier Services Ltd | Transport & logistics | 2025 |
| P & PKB Limited | Real estate | 2012 |
| P & R Property Investment Co. Ltd | Real estate | 2024 |
| P 'N' T Live Limited | Information & communication | 1997 |
| P Brennan Holdings Ltd | Financial services | 2024 |
| P Donaghey Plumbing & GAS Limited | Construction | 2012 |
| P Dutson Associates Limited | Other services | 2002 |
| P Dutson Consulting Ltd | Professional & scientific | 2018 |
| P H Dhanak Ltd | Health & social care | 2021 |
| P S Varsani Ltd | Health & social care | 2024 |
| P W Brennan Estates Ltd | Real estate | 2024 |
| P&D Residential Limited | Real estate | 2015 |
| P&S DE Silva Ltd | Health & social care | 2021 |
| P.& W.Kenton(Willesden)Limited | Real estate | 1949 |
| P.C Construction (UK) Ltd | Construction | 2011 |
| P.C.J Fast Food Limited | Wholesale & retail | 2012 |
| P.F.Sorici Ltd | Construction | 2010 |
| P1 Support-Services Limited | Information & communication | 2015 |
| P3O Consulting Solutions Ltd | Information & communication | 2014 |
| PA Companions Ltd | Health & social care | 2025 |
| PA Elkin Consultancy Limited | Financial services | 2022 |
| PA Mortgage & Protection Solutions Ltd | Financial services | 2026 |