| Octagram Group Limited | Professional & scientific | 2025 |
| OCTAV1 Limited | Wholesale & retail | 2016 |
| Oddies Bar Crewe Ltd | Hospitality | 2016 |
| Odourstream Limited | Construction | 2021 |
| Odykes Health Ltd | Health & social care | 2025 |
| OFF Your Head Ltd | Hospitality | 2022 |
| Offshore Wind Projects Ltd | Extraterritorial | 1999 |
| Ogarad Imaging Ltd | Health & social care | 2026 |
| OH My Glow HQ Ltd | Other services | 2020 |
| OHM Electrical Limited | Construction | 2024 |
| OJ Capital Ltd | Real estate | 2019 |
| OJU Healthcare Limited | Health & social care | 2016 |
| Olatun Ltd | Construction | 2023 |
| Old Pale Forestry | Extraterritorial | 2012 |
| Old Pale Future | Financial services | 2011 |
| Oldfile Services Limited | Real estate | 2026 |
| OLE Installations Ltd | Manufacturing | 2018 |
| OLH Finance Ltd | Professional & scientific | 2026 |
| Olive Nest Care Ltd | Health & social care | 2025 |
| Olive Spice Ltd | Hospitality | 2013 |
| Olive's Cypriot Street Food Ltd | Hospitality | 2021 |
| Olive's Mediterranean Food Limited | Hospitality | 2022 |
| Olivia Jayne Limited | Other services | 2024 |
| Ollerhead Holdings Limited | Real estate | 2022 |
| Olvossa Limited | Wholesale & retail | 2013 |
| Olympus Group Holdings Limited | Financial services | 2020 |
| Olympus International Limited | Wholesale & retail | 2009 |
| Olympus Recycling Limited | Information & communication | 2022 |
| OM Shanthaya Consultancy Ltd | Information & communication | 2026 |
| Ombwholistics Ltd | Administrative & support | 2025 |
| Omega Accounting Ltd | Professional & scientific | 2022 |
| Omega Pro Audio Ltd | Manufacturing | 2022 |
| OMG Experiences Limited | Wholesale & retail | 2019 |
| Omnia Consulting ACL Ltd | Professional & scientific | 2023 |
| OMR Sands Limited | Mining & quarrying | 1995 |
| ON Point Ventures Limited | Information & communication | 2018 |
| Onajite Cares Ltd | Health & social care | 2021 |
| One for ALL Supermarket Ltd | Wholesale & retail | 2022 |
| One Hundred Knights Limited | Professional & scientific | 2021 |
| ONE2GO Limited | Administrative & support | 2024 |
| Only Ceco Limited | Transport & logistics | 2016 |
| Only Edinburgh Ltd | Transport & logistics | 2024 |
| Only H.U.M.A.N. Consulting Ltd | Extraterritorial | 2024 |
| Onneley Golf Club (F&B) Ltd | Arts & recreation | 2025 |
| Onneley Golf Club Limited | Arts & recreation | 1998 |
| Onyx Lettings & Property Management Limited | Real estate | 2023 |
| OPA Property Solutions Limited | Real estate | 2025 |
| Opcreative Ltd | Professional & scientific | 2022 |
| Operational Cost Expert Ltd | Professional & scientific | 2017 |
| Opetech Global Services Limited | Real estate | 2020 |
| Opihc Properties Ltd | Real estate | 2026 |
| Opinion-Europe.Com Ltd | Professional & scientific | 2026 |
| Opti Technology Limited | Information & communication | 2024 |
| Optim Ltd | Wholesale & retail | 2019 |
| Optimal Evolution Limited | Professional & scientific | 2026 |
| Optimal Human Resources Limited | Professional & scientific | 2009 |
| Optimal-Planning Solutions Ltd | Professional & scientific | 2025 |
| Optimum Experience Ltd | Hospitality | 2021 |
| Optimum Pure Trade Ltd | Wholesale & retail | 2024 |
| Optimus Consulting (North West) Limited | Professional & scientific | 2021 |
| Opulent Baltic Jewellery Limited | Wholesale & retail | 2024 |
| Orange Dynasty Ltd | Real estate | 2018 |
| Orangepeel Interior Design Ltd | Professional & scientific | 2021 |
| Orbitas, Bereavement Services Limited | Construction | 2013 |
| Orchard Green Developments Limited | Construction | 2014 |
| Orchard Studios (Cheshire) Limited | Arts & recreation | 2002 |
| Orez Display Limited | Wholesale & retail | 2019 |
| Organic OAT Company Limited | Extraterritorial | 1986 |
| Oriental Daily Meal Ltd | Wholesale & retail | 2025 |
| Original Joinery Solutions Limited | Construction | 2016 |
| Orizont Recruiting Ltd | Administrative & support | 2025 |
| Orthohire Ltd | Wholesale & retail | 2015 |
| Oryn Interiors Limited | Professional & scientific | 2025 |
| Osafa Limited | Health & social care | 2025 |
| Osan Limited | Wholesale & retail | 1993 |
| Oscars Barbers Shop Limited | Other services | 2024 |
| Osiris Textiles LLP | | 2023 |
| OSL Global Limited | Construction | 2022 |
| OSO Natural Fuel Ltd | Extraterritorial | 2013 |
| Osprey Precision Ltd | Professional & scientific | 2016 |
| OSW Advisory LLP | | 2025 |
| OTZ Assessors Limited | Health & social care | 2020 |
| OUI Chef OUI Ltd | Hospitality | 2025 |
| OUR PUB Kink Limited | Hospitality | 2021 |
| OUR Space (Education) Ltd | Education | 2011 |
| OUT of Context Ltd | Wholesale & retail | 2026 |
| Outsourced Technical Solutions Ltd | Information & communication | 2011 |
| Outwell Limited | Professional & scientific | 1988 |
| OWL Farm Meat Ltd | Agriculture & fishing | 2026 |
| OWL Homes (Staffs) Ltd. | Real estate | 2019 |
| OWL Lettings Ltd | Real estate | 2021 |
| Owness Solutions Limited | Information & communication | 2013 |
| Oxford House Aesthetics Ltd | Other services | 2025 |
| Ozancik Catering Limited | Hospitality | 2019 |
| Ozzys Catering Limited | Hospitality | 2026 |
| Ozzyscrewe Limited | Hospitality | 2019 |
| O’ Keiths Property Investments Limited | Real estate | 2023 |
| P & J Kimpton Limited | Agriculture & fishing | 2022 |
| P & J Security & Electrical Systems Limited | Construction | 2004 |
| P & L Harrison Properties Ltd | Real estate | 2025 |