| Oligarch Commercial Property Ltd | Real estate | 2026 |
| Oligarch Residential Property Ltd | Households | 2026 |
| Oliver William Ltd | Real estate | 2025 |
| Oliver's Fish Bar Limited | Hospitality | 2021 |
| Ollybedhead Limited | Education | 2025 |
| Olton Boulevard Ltd | Hospitality | 2023 |
| Olympia Property Investments Limited | Real estate | 2019 |
| Olympia Sports Medicine Ltd | Health & social care | 2021 |
| Olympic Marine Limited | Administrative & support | 2010 |
| OM Anadi Ltd | Health & social care | 2021 |
| Omega Chemist Limited | Wholesale & retail | 2011 |
| Omega Research Labs Ltd | Manufacturing | 2025 |
| Omkara Productions Limited | Arts & recreation | 2015 |
| Omni Vest Properties Limited | Real estate | 2025 |
| Omnia Living Ltd | Real estate | 2021 |
| Omnia Restaurant Ltd | Hospitality | 2025 |
| Omnium Contracts Limited | Extraterritorial | 2022 |
| Omnium Holdings Ltd | Extraterritorial | 2022 |
| ON Business Ltd | Professional & scientific | 2000 |
| ON Trend Footwear Ltd | Extraterritorial | 2014 |
| ON2NET Holdings Limited | Real estate | 2021 |
| Once Upon a Dream Balloons Ltd | Hospitality | 2022 |
| One 4 ALL Trades Ltd | Manufacturing | 2003 |
| One Construction Group Ltd | Administrative & support | 2017 |
| One Dollar Mortimer Limited | Professional & scientific | 2019 |
| One E Solutions Ltd | Information & communication | 2003 |
| One Loyal Services Ltd | Education | 2021 |
| One Ninety Three Limited | Professional & scientific | 2007 |
| One Pima Ltd | Real estate | 2019 |
| One SIX O Limited | Professional & scientific | 2013 |
| One Sixty Four Communications Ltd | Professional & scientific | 2024 |
| One Stop Plus Limited | Wholesale & retail | 2025 |
| One TEN Ltd | Real estate | 2020 |
| One UBD Limited | Professional & scientific | 2026 |
| ONE2ONE Group Ltd | Health & social care | 2026 |
| Oneedge LLP | | 2009 |
| Onestop Van Solutions Ltd | Wholesale & retail | 2025 |
| Onethree Distribution Group Ltd | Transport & logistics | 2020 |
| Onethree Group Holdings Ltd | Financial services | 2023 |
| Oneworlddistribution Ltd | Wholesale & retail | 2018 |
| Onika Sabrina Ltd | Households | 2022 |
| Online Cloud Systems Limited | Information & communication | 2024 |
| Onlybros Ltd | Wholesale & retail | 2025 |
| Onwards Group C.I.C. | Health & social care | 2025 |
| Oodles Jobs Recruitment Limited | Administrative & support | 2022 |
| Opal Disposables Limited | Wholesale & retail | 2014 |
| Open Hearts & Minds Limited | Health & social care | 2025 |
| Open Path Supported Living Ltd | Health & social care | 2025 |
| Openbox Procurement Limited | Professional & scientific | 2016 |
| Operantis Limited | Information & communication | 2000 |
| Optcl Consulting Ltd | Information & communication | 2023 |
| Opti Communications Ltd | Professional & scientific | 2025 |
| Opti UP CIC | Education | 2025 |
| Optic.Sp Limited | Health & social care | 2018 |
| Optical Alarm Systems Ltd | Administrative & support | 2019 |
| Optical SH Ltd | Health & social care | 2025 |
| Optigreen Energy Ltd | Construction | 2025 |
| Optimal Graphics Ltd | Administrative & support | 2020 |
| Optimised Care Properties Limited | Public administration | 2025 |
| Optimum Solutions Ltd | Manufacturing | 2007 |
| Optimus Project & Cost Management Limited | Professional & scientific | 2014 |
| Opus Business Pensions Limited | Professional & scientific | 2015 |
| Opus Independent Financial Planning Limited | Professional & scientific | 2010 |
| Opus Property Management Limited | Real estate | 2015 |
| OR Property Developments Limited | Real estate | 2026 |
| Orange Archer Ltd | Wholesale & retail | 2022 |
| Orange Services Ltd | Information & communication | 2015 |
| ORB Telecom Ltd | Information & communication | 2006 |
| Orbital Bookings Ltd | Information & communication | 2025 |
| Orca Sports Management Limited | Other services | 2020 |
| Orchard County Systems Limited | Information & communication | 2007 |
| Orchard House (Midlands) Limited | Health & social care | 1986 |
| Orient Express Limited | Hospitality | 2014 |
| Orient Industrial Holdings Limited | Financial services | 2019 |
| Original Input Limited | Administrative & support | 2011 |
| Original Property Solutions Ltd | Real estate | 2024 |
| Orion Plumbing Limited | Construction | 2003 |
| Orivise Consulting Ltd | Professional & scientific | 2026 |
| Orthoserve Holdings Limited | Extraterritorial | 2009 |
| Orthoserve Limited | Manufacturing | 2010 |
| Ortus House Limited | Administrative & support | 2009 |
| OSB Consultancy UK Ltd | Professional & scientific | 2026 |
| Oscar Underhill Recruitment Solutions Limited | Administrative & support | 2023 |
| Oscott Academy Limited | Education | 2014 |
| Osdssdelt Ltd | Manufacturing | 2026 |
| Otec Trading Ltd | Wholesale & retail | 2022 |
| OTG Supplies Ltd | Wholesale & retail | 2026 |
| Otis Electrical Services Ltd | Manufacturing | 2023 |
| OUR Place Support C.I.C. | Information & communication | 2011 |
| OUR Property Hub Ltd | Real estate | 2018 |
| Outdoor Learning (UK) Limited | Education | 2015 |
| Outhouse Services Ltd | Administrative & support | 2025 |
| Outlet Kicks UK Limited | Wholesale & retail | 2025 |
| Ovation Support Ltd | Administrative & support | 2025 |
| Ovenstar (Midlands) Limited | Administrative & support | 2009 |
| Owen & Sons Investments Ltd | Real estate | 2023 |
| Owen House Holdings Ltd | Professional & scientific | 2020 |
| Owen Owen Renovations Limited | Construction | 2004 |
| Owens Mechanical & Machine Hire Limited | Construction | 2020 |
| OWH Ltd | Real estate | 2022 |