| Octopus Interiors Limited | Wholesale & retail | 2002 |
| ODS Studio Ltd | Information & communication | 2015 |
| Offley Memorials Limited | Other services | 2017 |
| Offshore Europe (Management) Limited | Administrative & support | 1988 |
| OH Honey House Ltd | Wholesale & retail | 2023 |
| Ohmz Ltd | Information & communication | 2018 |
| Ohsho Cosmetics Ltd | Wholesale & retail | 2019 |
| OIA Global Holdings (Europe) Limited | Transport & logistics | 2006 |
| OIA Global Limited | Other services | 1999 |
| OIL and GAS Services (Richmond) Limited | Mining & quarrying | 2025 |
| OIL Patch Logistics Ltd | Mining & quarrying | 2017 |
| Oisin Waddington Coaching Ltd | Education | 2024 |
| OLA T TM Ltd | Construction | 2023 |
| Old Deer Park Sports Ground Ltd | Other services | 2016 |
| Old Engineering Works Ltd | Real estate | 2020 |
| Old School Park Lane Limited | Households | 2023 |
| Oleificio Ltd | Manufacturing | 2025 |
| Olisie Limited | Professional & scientific | 2025 |
| Olive Branch Care and Support Ltd | Health & social care | 2024 |
| Olive Marketing Communications Limited | Professional & scientific | 2005 |
| Olive Residency Ltd | Real estate | 2022 |
| Oliver Anaesthesia Ltd | Administrative & support | 2017 |
| Oliver Cope Limited | Professional & scientific | 2025 |
| Oliver's Island Limited | Information & communication | 2010 |
| Olivia Flattery HR Limited | Administrative & support | 2015 |
| OLK Property Limited | Real estate | 2017 |
| Ollard Consulting Limited | Professional & scientific | 2018 |
| Olvaro Ltd | Information & communication | 2025 |
| Olympus Legal Ltd | Professional & scientific | 2023 |
| Olympus Sports Ltd | Arts & recreation | 2017 |
| Omega Capital Limited | Professional & scientific | 2006 |
| Omel Group Ltd | Construction | 2024 |
| Omicsure Ltd | Information & communication | 2025 |
| Omnai Marketing Ltd | Professional & scientific | 2024 |
| Omni Airline Services Group Limited | Financial services | 2024 |
| Omnilateral Limited | Information & communication | 2003 |
| Omnisent Limited | Information & communication | 2025 |
| OMP Automotive Limited | Wholesale & retail | 2020 |
| Omurtag Thetford 2 Ltd | Real estate | 2025 |
| Omut Capital, LLC | | 2024 |
| ON Course Foundation | Arts & recreation | 2010 |
| ON Sale Group Ltd | Professional & scientific | 2024 |
| ON the Ground Communications Ltd | Professional & scientific | 2024 |
| ONBOARD42 Limited | Information & communication | 2025 |
| Onbooktheatre Ltd | Arts & recreation | 2023 |
| Ondula Ltd | Professional & scientific | 2025 |
| One & Only Properties Limited | Real estate | 2019 |
| One MAX Limited | Information & communication | 2008 |
| One Paved Court Ltd | Wholesale & retail | 2017 |
| One Retail Ltd | Other services | 2023 |
| One Stop Supplies Direct Limited | Wholesale & retail | 2024 |
| One TEN Consulting Limited | Information & communication | 2023 |
| Online Elusive Ltd | Wholesale & retail | 2025 |
| Only the Best Productions Limited | Information & communication | 2013 |
| Onslow Court Management (Richmond) Limited | Households | 1982 |
| Oops Consultancy Limited | Professional & scientific | 1997 |
| Openwide Coastal Limited | Arts & recreation | 2008 |
| Openwide International Limited | Arts & recreation | 2022 |
| Opex Solutions Limited | Professional & scientific | 2011 |
| OPM Developments Limited | Real estate | 2020 |
| OPS Richmond Ltd | Real estate | 2025 |
| Optara Limited | Information & communication | 2023 |
| Optimaland Limited | Construction | 2005 |
| Optimistic Holdings Limited | Financial services | 2025 |
| Opus Animae Consulting Limited | Administrative & support | 2016 |
| Oqtonom Ltd | Information & communication | 2026 |
| Orange Finance Limited | Professional & scientific | 2023 |
| Orange Malone Ltd | Administrative & support | 2005 |
| Orange Packaging Ltd | Wholesale & retail | 2021 |
| Orange Three Ltd | Administrative & support | 2024 |
| Orchard Building Solutions Ltd | Construction | 2009 |
| Orchard Fire Protection Limited | Public administration | 2022 |
| Orchard Mews (Scotton) Management Company Limited | Households | 1993 |
| Ordinum Enterprises Limited | Information & communication | 2017 |
| Organic Passion Ltd | Wholesale & retail | 2022 |
| Oriana Consultancy Ltd | Professional & scientific | 2026 |
| Oriental Thai Cuisine Ltd | Hospitality | 2016 |
| Origin Active Living (UK) Ltd | Financial services | 2022 |
| Origin Build Ltd | Real estate | 2022 |
| Origin Energy CIC | Energy | 2007 |
| Origin Structures Limited | Professional & scientific | 2004 |
| Orion Consulting Group Limited | Professional & scientific | 2016 |
| Orsman Whitmore Limited | Real estate | 1998 |
| Orsman Whitmore Property Management Ltd | Real estate | 2013 |
| Orvian Concierge Ltd | Transport & logistics | 2026 |
| Osborne Long Ltd | Professional & scientific | 2017 |
| Osborne Martin Holdings Ltd | Real estate | 2025 |
| Oscar Sandhu Investments Limited | Real estate | 2023 |
| Osdy Limited | Professional & scientific | 2025 |
| Osiris Law Limited | Professional & scientific | 2013 |
| Ospet Investment Limited | Real estate | 2024 |
| Osterley Catering Ltd | Hospitality | 2016 |
| OTC Scoop.Com Limited | Information & communication | 2000 |
| Otto Property Investments Ltd | Real estate | 2021 |
| Ottomater Ltd | Information & communication | 2020 |
| OUR Local Plumber Ltd | Construction | 2020 |
| OUR Riverside Cottage Company Ltd | Manufacturing | 2023 |
| Ourweb Ltd | Administrative & support | 2011 |
| OUT of Town Limited | Real estate | 2025 |
| Outdoor Financial Ltd | Information & communication | 2023 |