| Onscreencounselling Limited | Health & social care | 2014 |
| Onshore Energy Conference Limited | Administrative & support | 2015 |
| Onslow Square (SP) Limited | Real estate | 2023 |
| Ontech Communications Limited | Construction | 2020 |
| Ontologi Limited | Information & communication | 2021 |
| Onward Solutions (2019) Ltd | Professional & scientific | 2019 |
| Ooda Limited | Professional & scientific | 2014 |
| Oooh Balloons Limited | Arts & recreation | 2017 |
| Oosthuizen Consulting Limited | Professional & scientific | 2025 |
| Opal (UK) Holdings Limited | Professional & scientific | 2003 |
| Opal Information Systems Limited | Professional & scientific | 1994 |
| Open Living (UK) Ltd | Construction | 2011 |
| Open Source Ltd | Manufacturing | 2026 |
| Open Space Construction Ltd | Construction | 2023 |
| Openai 10X Opportunity LP | | 2024 |
| Opening City Doors Ltd. | Professional & scientific | 2014 |
| Operandi Consulting Ltd | Professional & scientific | 2023 |
| OPI and Orlando Limited | Wholesale & retail | 2022 |
| Optima Management Consultants Limited | Professional & scientific | 2012 |
| Optima Publishing (2012) Ltd | Information & communication | 2012 |
| Optima-Life Limited | Health & social care | 2007 |
| Optima-Life Therapies Limited | Health & social care | 1999 |
| Optional Intelligence Limited | Information & communication | 2012 |
| Options Property (Maidenhead) Limited | Real estate | 2019 |
| Optipod Ltd | Manufacturing | 2026 |
| Optometron Ltd | Real estate | 2021 |
| Opulent Assets and Reality Ltd | Real estate | 2024 |
| Opus Sterling Recruitment Limited | Administrative & support | 2013 |
| Orange Core Aluminium Windows and Doors Limited | Construction | 2013 |
| Orangecore Glass & Windows Limited | Administrative & support | 2023 |
| Orangecore Manufacture/Installers 0F Upvc/Aluminimum Windows & Doors Glass Sealed Units Limited | Manufacturing | 2023 |
| ORB Management Limited | Administrative & support | 2013 |
| Orchard Clinic Limited | Health & social care | 1997 |
| Orchardwise Limited | Professional & scientific | 2016 |
| Orchid House (St.Albans) Management Company Limited | Households | 1984 |
| Orchid Services Limited | Construction | 2021 |
| Organise IT Ltd | Information & communication | 2021 |
| Orhan Barber Ltd | Other services | 2024 |
| Orient Close Nursery Limited | Other services | 2009 |
| Orient Regeneration | Professional & scientific | 1997 |
| Oriental Pearl Spa Ltd | Other services | 2026 |
| Origami Accounting and Advisory Limited | Professional & scientific | 2025 |
| Origin AV Ltd | Arts & recreation | 2026 |
| Origin Cleaning Ltd | Water & waste | 2000 |
| Origin Drainage Services Ltd | Water & waste | 2012 |
| Original Beans UK Limited | Wholesale & retail | 2022 |
| Oril'Snicanor Ltd | Professional & scientific | 2023 |
| Orion 76 Limited | Transport & logistics | 2026 |
| Orion Vista Creative Marketing Ltd | Professional & scientific | 2025 |
| Orkestrate Ltd | Professional & scientific | 2020 |
| Orlaroko Properties Ltd | Real estate | 2020 |
| Ormarella Limited | Administrative & support | 2016 |
| Ormonde Gate Property Limited | Real estate | 2023 |
| Osbourne Heights Warley Limited | Households | 2005 |
| Oscera Ltd | Health & social care | 2026 |
| Osir Company Limited | Wholesale & retail | 2023 |
| Osprey Homes (March) Limited | Real estate | 2019 |
| Ossey Limited | Manufacturing | 2020 |
| Ostmodern Ltd | Information & communication | 2007 |
| OT and Beyond C.I.C. | Education | 2025 |
| Other Hand Media Ltd | Information & communication | 2026 |
| Otis Properties Ltd | Real estate | 2024 |
| OTM Optics Ltd | Administrative & support | 2022 |
| Otman's Driving School Ltd | Education | 2025 |
| OTR Labour Solutions Ltd | Administrative & support | 2018 |
| Ottiva Limited | Construction | 2010 |
| OUR Choice Online Limited | Wholesale & retail | 2016 |
| OUR Lady's Abingdon Trustees Limited | Education | 2007 |
| Outdoor Modular Spaces Limited | Manufacturing | 2015 |
| Outdoors Works Ltd | Construction | 2026 |
| Outline Engineering Ltd | Professional & scientific | 2024 |
| Output House Ltd | Information & communication | 2022 |
| Outrun SPV I LLP | | 2021 |
| Outrun SPV V LLP | | 2022 |
| Outside in Construction Limited | Construction | 2013 |
| Outsource Admin Ltd | Administrative & support | 2025 |
| Outsourced Professional Administration Limited | Professional & scientific | 1994 |
| OV Quality & Compliance Ltd | Administrative & support | 2016 |
| Oval Ball Consulting Ltd | Professional & scientific | 2024 |
| Overland Escapes Limited | Administrative & support | 2025 |
| Overseas Corporate Services Limited | Administrative & support | 2003 |
| Overseas Footwear Trading Corporation Limited | Professional & scientific | 1964 |
| OVO Theatre Limited | Arts & recreation | 2023 |
| OWL Vent Ltd | Construction | 2023 |
| Own IT Home Ltd | Professional & scientific | 2019 |
| Oxford Bioresearch and Education Ltd | Health & social care | 2010 |
| Oxford Marketing Ltd | Education | 2002 |
| Oxygen Retail Limited | Wholesale & retail | 2008 |
| O`Loyd Consulting Limited | Professional & scientific | 2007 |
| P & B Woodwork Ltd | Construction | 2009 |
| P & S Enterprises (London) Limited | Real estate | 2016 |
| P G Creative Ltd | Information & communication | 2023 |
| P Gallagher Construction Ltd | Construction | 2022 |
| P J Casper Associates Limited | Professional & scientific | 2013 |
| P J Mccarthy Landscapes Limited | Construction | 2013 |
| P M Fernando Ltd | Health & social care | 2012 |
| P R Gray Ltd. | Extraterritorial | 2013 |
| P Rayner Consulting Limited | Information & communication | 2016 |
| P&P Chavda LLP | | 2023 |
| P+R Investments Limited | Construction | 2003 |