| Oraculi Limited | Professional & scientific | 2020 |
| Orange Assets Bidco 2 Limited | Financial services | 2026 |
| Orange Assets Bidco Limited | Financial services | 2025 |
| Orange Assets Holdco 2 Limited | Financial services | 2026 |
| Orange Assets Holdco Limited | Financial services | 2025 |
| Orange Investments One Limited | Professional & scientific | 2014 |
| Orange Partners Limited | Professional & scientific | 2023 |
| Orange Racing Limited | Arts & recreation | 2015 |
| Orange UK Holdings Limited | Professional & scientific | 2017 |
| Orange UK Property Investments Limited | Real estate | 2022 |
| Orangeflower Limited | Wholesale & retail | 2025 |
| Orangeries and Solid Roofs Limited | Construction | 2017 |
| Orbinova Solutions Limited | Information & communication | 2025 |
| Orbis Ventures Limited | Financial services | 2024 |
| Orbsen Therapeutics UK Ltd | Professional & scientific | 2017 |
| Orca Marketplace Ltd | Other services | 2026 |
| Orchard Cottage Developments Limited | Real estate | 2023 |
| Orchard Court (Bury) Management Limited | Households | 2004 |
| Orchard END Diagnostics Ltd | Health & social care | 2023 |
| ORCHESTR8 Digital Ltd | Information & communication | 2018 |
| Ordo Europe Limited | Wholesale & retail | 2023 |
| Ordo Holdco Limited | Professional & scientific | 2019 |
| Ordo IP Limited | Administrative & support | 2023 |
| Ordoclub Ltd | Administrative & support | 2017 |
| ORG 3D Search Ltd | Professional & scientific | 2018 |
| Organic Waste Management (ST Helens) Ltd | Water & waste | 2005 |
| Organic Waste Management Limited | Water & waste | 2004 |
| Organics QBD Limited | Professional & scientific | 2024 |
| Organisation Consulting Limited | Administrative & support | 2023 |
| Orient Supermarket Limited | Wholesale & retail | 2017 |
| Origin Furniture Limited | Manufacturing | 2005 |
| Original Property Investments Limited | Real estate | 2021 |
| Orinoco Limited | Professional & scientific | 2024 |
| Orion ALS Consulting Limited | Professional & scientific | 2024 |
| Orotava Properties Ltd | Real estate | 2021 |
| Orphic Design Limited | Information & communication | 2018 |
| Ortho Ceras Ltd. | Professional & scientific | 2012 |
| Orthocorp Ltd | Health & social care | 2022 |
| Orthopets Europe Limited | Professional & scientific | 2008 |
| Orthteam (Didsbury) Limited | Health & social care | 2017 |
| Ortolan Limited | Professional & scientific | 2024 |
| Ortus UK Ltd | Wholesale & retail | 2008 |
| Orynex Engineering Ltd | Manufacturing | 2026 |
| Osalahuddin Ltd | Real estate | 2022 |
| Osata Innovation Ltd | Professional & scientific | 2025 |
| OSB 2016 Limited | Administrative & support | 2016 |
| OSC Site Services Ltd | Manufacturing | 2017 |
| Oscar & Webb Ltd | Administrative & support | 2018 |
| Oscar App Limited | Information & communication | 2025 |
| Oscy Estates Limited | Financial services | 2025 |
| OSD Consulting Limited | Information & communication | 2024 |
| Oserth UK Ltd | Manufacturing | 2025 |
| Ospero Limited | Professional & scientific | 2011 |
| Ossia Ltd | Wholesale & retail | 2020 |
| OSW Limited | Administrative & support | 2012 |
| Otter Products UK Limited | Administrative & support | 2014 |
| Otto Advisory Group Ltd | Information & communication | 2024 |
| Otto Cyber Security Limited | Information & communication | 2023 |
| Otto Insure Limited | Financial services | 2020 |
| Otto Wines Limited | Hospitality | 2023 |
| Otus Financial Ltd | Financial services | 2011 |
| Otwt Limited | Hospitality | 2018 |
| OUR Deli Limited | Wholesale & retail | 2019 |
| Outdoor Living by Signature Landscapes Ltd | Construction | 2024 |
| Outerlimit Group Ltd | Information & communication | 2024 |
| Outerlimit IP Ltd | Information & communication | 2025 |
| Outland Estates Ltd | Construction | 2016 |
| Outside Context Development Limited | Administrative & support | 2016 |
| Outsider Intelligence Ltd | Information & communication | 2026 |
| Outsource B2B Limited | Administrative & support | 2021 |
| Ovaici Medical Ltd | Health & social care | 2018 |
| Ovalgate Properties Limited | Real estate | 2011 |
| Ovenmitz Limited | Hospitality | 2001 |
| Overarching IT Limited | Administrative & support | 2015 |
| Overland Specialist Vehicles Ltd | Wholesale & retail | 2019 |
| Owen Clinical Consulting Limited | Professional & scientific | 2016 |
| Own WON NOW Limited | Information & communication | 2024 |
| Owton Limited | Administrative & support | 2013 |
| Oxford Apparel Imports Limited | Financial services | 2010 |
| Oxford Decisions Limited | Administrative & support | 2014 |
| Oxoid (ELY) Limited | Professional & scientific | 1987 |
| Oxoid 2000 Limited | Professional & scientific | 2000 |
| Oxoid Holdings Limited | Professional & scientific | 1996 |
| Oxoid International Limited | Manufacturing | 1996 |
| Oxoid Limited | Manufacturing | 1996 |
| Oxoid Senior Holdings Limited | Professional & scientific | 2004 |
| Oyster Images Limited | Other services | 2008 |
| OZA Investments Limited | Real estate | 2024 |
| Ozon Enterprises Limited | Financial services | 2016 |
| Ozon Solicitors Limited | Professional & scientific | 2010 |
| Ozric AI Limited | Information & communication | 2026 |
| P & B Electrical Ltd | Professional & scientific | 2020 |
| P & E Plumbing & Heating Services Limited | Construction | 2001 |
| P & F Allman Ltd | Wholesale & retail | 2010 |
| P & J Collins Contractors Limited | Construction | 2005 |
| P a Williams Limited | Administrative & support | 2015 |
| P and C Retail UK Limited | Wholesale & retail | 2009 |
| P and D Trading Limited | Wholesale & retail | 2014 |
| P and T Construction Limited | Administrative & support | 2016 |
| P H Investments (NO 2) Limited | Administrative & support | 2010 |