| OE Parks Limited | Real estate | 2025 |
| OF Properties Ltd | Real estate | 2023 |
| Offersafe Limited | Extraterritorial | 2020 |
| Office App International Ltd | Financial services | 2016 |
| Officers Colours Ltd | Wholesale & retail | 2005 |
| Official EM Rose Limited | Information & communication | 2022 |
| Offset Design Ltd | Information & communication | 2013 |
| OH Brother Bagels Limited | Manufacturing | 2025 |
| Okra Holdings Limited | Administrative & support | 2018 |
| Okra Technologies Limited | Information & communication | 2015 |
| Oladavid Consultancy Services Limited | Professional & scientific | 2014 |
| Old Manor Estates Limited | Real estate | 1993 |
| Old Manor Partnership Limited | Agriculture & fishing | 2020 |
| Old Mill House Limited(The) | Households | 1978 |
| Old Mill Lodge Limited | Hospitality | 2019 |
| Old Yard Close Management Ltd | Real estate | 2000 |
| Olive Printing Services Limited | Information & communication | 2026 |
| Oliver Forbes-Turner Ltd | Health & social care | 2010 |
| Oliver Joshi Ltd | Other services | 2017 |
| Olympex Holdings Ltd | Financial services | 2021 |
| Omarca Investment Holdings Limited | Professional & scientific | 1988 |
| OMC Investments Limited | Administrative & support | 1970 |
| OMC Medical Limited | Professional & scientific | 2020 |
| Omega Hot TUB Servicing Ltd | Manufacturing | 2018 |
| OMG Yolo Ltd | Hospitality | 2022 |
| Omni Source Ltd | Wholesale & retail | 2026 |
| OMNI3 Limited | Construction | 2015 |
| Omnient Limited | Information & communication | 2025 |
| Omnipearl Ltd | Professional & scientific | 2012 |
| ON Merit Ltd | Administrative & support | 2025 |
| ON Point Events Ltd | Administrative & support | 2021 |
| ON Point Media Sales Limited | Professional & scientific | 2016 |
| ON the Spot Accountants Limited | Extraterritorial | 2026 |
| ON the Spot Tax Limited | Professional & scientific | 2008 |
| On-Line Partnership Investments Limited | Financial services | 2023 |
| Onabox Ltd | Construction | 2019 |
| Onawall Ltd | Construction | 2025 |
| Onco-It Consulting Ltd | Information & communication | 2020 |
| One Life International | Health & social care | 2001 |
| One MAN Studio Ltd | Information & communication | 2021 |
| One Mortgage System Limited | Information & communication | 2017 |
| One Source Finance Limited | Financial services | 2024 |
| One TEN Digital Limited | Information & communication | 2011 |
| Oneirion Digital Limited | Information & communication | 2026 |
| Onepdm Consulting Limited | Professional & scientific | 2023 |
| Onetrak Limited | Construction | 2018 |
| Online Travel Training Limited | Information & communication | 2023 |
| Onyangs Ltd | Wholesale & retail | 2023 |
| Onyx Motor Company Limited | Wholesale & retail | 2024 |
| Onyx Vans Limited | Manufacturing | 2022 |
| Opal Partners (Europe) LLP | | 2022 |
| Open Pathways Psychotherapy Ltd | Health & social care | 2026 |
| Open Road Driving Academy Limited | Education | 2016 |
| Open UK JOB Ltd | Education | 2025 |
| Open UK Properties Ltd | Real estate | 2025 |
| Open UK Study Ltd | Education | 2022 |
| Openside Business Solutions Limited | Information & communication | 2013 |
| Operative Consulting Limited | Professional & scientific | 2023 |
| OPS Coach Ltd | Education | 2025 |
| Optic Laboratory Ltd | Health & social care | 2011 |
| Optimal Car Detailers Ltd | Wholesale & retail | 2026 |
| Optimo Events Ltd | Administrative & support | 2017 |
| Optimum HR Limited | Administrative & support | 2020 |
| Optisense Limited | Manufacturing | 1998 |
| Opus Living Limited | Real estate | 2024 |
| Orange and Green Company Limited | Wholesale & retail | 2012 |
| Orange Submarine Limited | Information & communication | 2026 |
| Orange Technical Productions Limited | Information & communication | 2017 |
| Orange Tree Farm Ltd | Agriculture & fishing | 2020 |
| ORB Heating Ltd | Construction | 2023 |
| Orchard Brae Extra Care Limited | Construction | 2025 |
| Organic Lifestyle Limited | Other services | 2001 |
| Organicchildren Limited | Extraterritorial | 2001 |
| Origin - Return to Eden Ltd | Manufacturing | 2024 |
| Original Mushroom Company Limited | Agriculture & fishing | 2020 |
| Orion Confectionery Ltd | Wholesale & retail | 2009 |
| Orla Constant Ltd | Professional & scientific | 2013 |
| Ornate Fabrications Limited | Manufacturing | 2020 |
| Orpheus Publishing Limited | Information & communication | 2021 |
| ORS INT Ltd | Transport & logistics | 2017 |
| Oryx Fidelitas Limited | Information & communication | 2014 |
| OS Agri and Fencing Limited | Agriculture & fishing | 2025 |
| Osbrooks Farm Storage Limited | Transport & logistics | 2023 |
| Ostendfresh (UK) Limited | Transport & logistics | 2020 |
| Osterley House Management Company Limited | Households | 2007 |
| Ottilie M Limited | Real estate | 2019 |
| Out-Sourcing Initiatives Limited | Professional & scientific | 2004 |
| Outdoor Event Equipment Hire Limited | Administrative & support | 2015 |
| Outlw Ltd | Wholesale & retail | 2020 |
| Outside the Bubble Ltd | Professional & scientific | 2022 |
| Ouul Holdings Ltd | Financial services | 2025 |
| Ouul Sports Ltd | Manufacturing | 2025 |
| Oval Securities Limited | Real estate | 1997 |
| Oveedee Ltd | Wholesale & retail | 2026 |
| Oven Heaven Limited | Administrative & support | 2009 |
| Over Proof Communications Ltd | Professional & scientific | 2023 |
| Overwatch Risk Management Ltd | Professional & scientific | 2019 |
| Oxford Alpha Limited | Information & communication | 2009 |
| Oxford Road (Horsham) Management Company Limited | Households | 2009 |
| Oxford Road Horsham (Flats) Residents Association Limited | Households | 2009 |