| Oslo Airport Transfer Ltd | Transport & logistics | 2026 |
| OSM Building Services Ltd | Professional & scientific | 2022 |
| OSM Investment Ventures Limited | Financial services | 2024 |
| Osmani Logistik Ltd. | Information & communication | 2006 |
| Osmosis Flora Limited | Wholesale & retail | 2011 |
| OSN Properties Limited | Real estate | 2025 |
| OSO Content Studio Limited | Other services | 2025 |
| OSP Investment Holdings Limited | Financial services | 2026 |
| OSP Quantum Limited | Professional & scientific | 2020 |
| OSR Projects Limited | Real estate | 2016 |
| Ostanska Consulting Ltd | Wholesale & retail | 2016 |
| Osterbay Limited | Hospitality | 2021 |
| OT Properties & Investments Ltd | Households | 2025 |
| OT Tutors Limited | Education | 2024 |
| Otakustudiogroup Ltd | Information & communication | 2025 |
| OTC Investments Ltd | Real estate | 2025 |
| Otec North Ltd | Wholesale & retail | 2022 |
| Otech Project Management Limited | Administrative & support | 2014 |
| Otech Properties Ltd | Real estate | 2020 |
| Othello Trading Limited | Energy | 2012 |
| Otherwise Productions Ltd | Information & communication | 2025 |
| Otherworld Entertainment Limited | Information & communication | 2022 |
| Othisi Ltd | Administrative & support | 2023 |
| Othman Heart & Aortic Solutions Ltd | Health & social care | 2025 |
| Othman Properties Ltd | Real estate | 2022 |
| OTI Tilly Ltd | Arts & recreation | 2025 |
| Otilia Banica Ltd | Health & social care | 2025 |
| Otis Property Limited | Real estate | 2018 |
| Otoko Designs Limited | Administrative & support | 2018 |
| Otterspool Adventure Centre Limited | Arts & recreation | 2009 |
| Otterspool Management Company Limited | Households | 1989 |
| Otterspool Presents Ltd | Hospitality | 2025 |
| Ottilie One Ltd | Hospitality | 2025 |
| Ottokunst Ltd | Wholesale & retail | 2023 |
| Ottoman Property Management Limited | Real estate | 2013 |
| Ottoman's Social Complex | Other services | 2015 |
| OUR Fade Hair Style Ltd | Other services | 2025 |
| OUR Homes Group Holdings Limited | Real estate | 2022 |
| OUR Place Community Hub CIC | Health & social care | 2023 |
| OUR Village Homes Ltd | Real estate | 2025 |
| Ours Support Ltd | Health & social care | 2025 |
| OUS Group Ltd | Arts & recreation | 2016 |
| Ouse Ltd | Wholesale & retail | 2023 |
| OUT of Hours Kids' Club Limited | Administrative & support | 1999 |
| OUT of Phase Events Limited | Other services | 2021 |
| OUT of Time (UK) Ltd | Wholesale & retail | 2009 |
| OUT There Media Distribution Ltd | Professional & scientific | 2022 |
| OUT to Lunch (Liverpool) Limited | Wholesale & retail | 2006 |
| OUT to Lunch Limited | Hospitality | 2013 |
| Outcast in Love Ltd | Wholesale & retail | 2023 |
| Outcider Limited | Information & communication | 2011 |
| Outdistance Limited | Professional & scientific | 2004 |
| Outdoor Abode Ltd | Wholesale & retail | 2023 |
| Outdoor-Maintenance Northwest Ltd | Administrative & support | 2024 |
| Outer Waves Ltd | Arts & recreation | 2024 |
| Outlast Global Holdings Co., Ltd | Wholesale & retail | 2025 |
| Outlier Developments Ltd | Construction | 2020 |
| Outlier Strategy Ltd | Other services | 2018 |
| Outofthisworldkidsclub Limited | Health & social care | 2011 |
| Outreach IT Limited | Real estate | 2004 |
| Outreal Interactive Entertainment Ltd | Arts & recreation | 2022 |
| Outright Brickwork Ltd | Construction | 2017 |
| Outsource Consultancy Limited | Other services | 2017 |
| Outsource PAY Genie Ltd | Financial services | 2023 |
| Outsourcing Payroll Umbrella Solutions Ltd | Professional & scientific | 2022 |
| Outwood Surveyors Ltd | Professional & scientific | 2009 |
| Oval (2143) Limited | Real estate | 2007 |
| Ovantra Limited | Real estate | 2025 |
| Ovation Creative Management Ltd | Administrative & support | 2025 |
| Over the Bridge (Liverpool) Limited | Hospitality | 2011 |
| Over the Influence (OTI) Limited | Arts & recreation | 2022 |
| Overchurch Building & Commercial Property Limited | Construction | 2013 |
| Overdale Farm (Neston) Limited | Construction | 2023 |
| Overlay Labs Ltd | Information & communication | 2026 |
| Overly Dedicated Agency Ltd | Wholesale & retail | 2020 |
| Overseas Critical Care, Emergency Medicine, and Anaesthesia Network (Ocean) Ltd | Health & social care | 2025 |
| Overshoot Productions Ltd | Information & communication | 2024 |
| Oversight Labs Limited | Information & communication | 2026 |
| Overtidy Limited | Construction | 1992 |
| Overture Developments Ltd | Real estate | 2004 |
| Overture Theatre Ltd | Arts & recreation | 2026 |
| Overwatch Consultancy UK Ltd | Professional & scientific | 2025 |
| Overwrite Ltd | Administrative & support | 2022 |
| Ovesca Energy Limited | Professional & scientific | 2012 |
| Ovia Group Ltd | Professional & scientific | 2024 |
| Ovidiu Services Ltd | Health & social care | 2023 |
| Ovolu Ltd | Information & communication | 2022 |
| Owch Limited | Information & communication | 2024 |
| Owen Bros. (Footwear) Limited | Manufacturing | 1949 |
| Owen Drew Luxury Candles Limited | Manufacturing | 2016 |
| Owen Ellis Partnership Limited(The) | Professional & scientific | 1986 |
| Owen Rail Limited | Construction | 2019 |
| Owenco 1 Limited | Extraterritorial | 2017 |
| Owens & Partners Ltd | Construction | 2024 |
| Owens Houses Ltd | Real estate | 2026 |
| Owens Performance Solutions Ltd | Professional & scientific | 2022 |
| Owens Specialized Builders Ltd | Construction | 2025 |
| Owens Subsea Ltd | Professional & scientific | 2026 |
| OWL & Lizard C.I.C. | Agriculture & fishing | 2023 |
| OWL Tree A.S. Ltd | Education | 2025 |