| Ockham Biotech Limited | Professional & scientific | 2006 |
| OCR Executive Limited | Administrative & support | 2019 |
| OCT Films Ltd | Information & communication | 2011 |
| OCT Marine LLP | | 2012 |
| OCT Properties Limited | Real estate | 2020 |
| Octarine IT Limited | Professional & scientific | 2025 |
| October Consultancy Portsmouth Limited | Professional & scientific | 2025 |
| Octoginta Septem Ltd | Professional & scientific | 2018 |
| Ocupa Ltd | Health & social care | 2022 |
| OD Craft Bars Limited | Hospitality | 2022 |
| Odin Stones Limited | Other services | 2024 |
| Odonata Group Ltd | Agriculture & fishing | 2022 |
| Oeil Ltd | Other services | 2018 |
| OF Books and Love Limited | Wholesale & retail | 2025 |
| OF Lands FAR Away Limited | Wholesale & retail | 2024 |
| OFF Duty Ltd | Wholesale & retail | 2024 |
| Office Bison Limited | Wholesale & retail | 2025 |
| Office Limited | Public administration | 2025 |
| Official Insight Consultancy Ltd | Professional & scientific | 2022 |
| Offshore Marine Technology Limited | Manufacturing | 2024 |
| Ogden M (TT) Ltd | Wholesale & retail | 2005 |
| OGW Properties Ltd | Real estate | 2018 |
| OHM Sweet OHM Limited | Real estate | 2022 |
| OIL and GAS Resourcing Ltd | Mining & quarrying | 2012 |
| OJC Design and Build Limited | Construction | 2026 |
| Okehampton Specsavers Limited | Wholesale & retail | 2002 |
| Okemsidera Holding Limited | Financial services | 2024 |
| Okotuna Limited | Extraterritorial | 2015 |
| Okra Lounge Ltd | Hospitality | 2022 |
| OKW Global Ltd | Manufacturing | 2025 |
| Old Link House Limited | Real estate | 1977 |
| Old Skool Events UK Ltd | Hospitality | 2022 |
| Old Swan Specsavers Limited | Wholesale & retail | 2007 |
| Old Swan Visionplus Limited | Wholesale & retail | 2007 |
| Oldham Domiciliary Specsavers Limited | Wholesale & retail | 2017 |
| Oldham Specsavers Hearcare Limited | Wholesale & retail | 2006 |
| Olip Therapy Ltd | Health & social care | 2005 |
| Olive & Friends Limited | Real estate | 2024 |
| Olive Business Solutions Limited | Information & communication | 2003 |
| Olive Tree Accounting Limited | Professional & scientific | 2011 |
| Oliver Bennett High Quality Decorating Services Limited | Construction | 2016 |
| Oliver Effects Limited | Wholesale & retail | 2025 |
| Oliver Henry Enterprise Limited | Information & communication | 2025 |
| Oliver James Property Maintenance Limited | Professional & scientific | 2024 |
| Oliver's Rainbow Nursery Ltd | Health & social care | 2024 |
| Olivers of Southampton Limited | Wholesale & retail | 2002 |
| Ollited Limited | Health & social care | 2014 |
| Olympian Holiday Homes Ltd | Real estate | 2018 |
| Omeen Management Limited | Professional & scientific | 2025 |
| Omega Burger Ltd | Extraterritorial | 2026 |
| Omega Elifar Limited | Health & social care | 2006 |
| Omkar Enterprises Ltd | Hospitality | 2026 |
| Omnichat.Uk Ltd | Information & communication | 2024 |
| Omnitix Ticketing Limited | Professional & scientific | 2026 |
| ON Form Personal Training Limited | Health & social care | 2017 |
| ON Point Electrical Services Limited | Construction | 2012 |
| ON Virgin Territory Limited | Administrative & support | 2004 |
| Onboard Projects Limited | Manufacturing | 2023 |
| One Cloud at a Time Events and Adventures Limited | Administrative & support | 2025 |
| One Cloud Telecom Ltd. | Information & communication | 2019 |
| One Eighth Limited | Hospitality | 2024 |
| One MSL Limited | Education | 2017 |
| One Nigel Ltd | Information & communication | 2020 |
| One Point Five Ltd | Professional & scientific | 2018 |
| One World One Pitch CIC | Professional & scientific | 2024 |
| Onecloud Limited | Extraterritorial | 2016 |
| Onecom Limited | Information & communication | 2000 |
| Onecom Partners Limited | Information & communication | 2020 |
| Onecom Technology Group Limited | Other services | 2012 |
| Onedome Capital Ltd | Professional & scientific | 2011 |
| Onedome Finance Ltd | Financial services | 2022 |
| Onepoundwebhosting Limited | Information & communication | 2006 |
| Onestream Limited | Information & communication | 2016 |
| Online Resources Ltd | Information & communication | 2010 |
| Only G.O.A.T Limited | Other services | 2023 |
| Onlycerts Ltd | Professional & scientific | 2024 |
| Onsaco Ltd | Wholesale & retail | 2017 |
| Onyx Developments (Southern) Ltd | Construction | 2023 |
| Onyx Renovations Ltd | Construction | 2025 |
| Opal Dental Studios Limited | Health & social care | 2025 |
| Opal FEE Recovery Ltd | Administrative & support | 2023 |
| Opal Legal Ltd | Professional & scientific | 2024 |
| Opal Packaging UK Limited | Administrative & support | 2024 |
| Open Access Healthcare Limited | Health & social care | 2023 |
| Open Air Padel Co Limited | Arts & recreation | 2026 |
| Open Circles Arts C.I.C. | Arts & recreation | 2018 |
| Open Oven Pizza Company Limited | Other services | 2013 |
| Open the POD Bay Doors Ltd | Information & communication | 2025 |
| Openshaw Specsavers Limited | Wholesale & retail | 2016 |
| Ophir Road RTM Company Limited | Households | 2021 |
| Optic Networx Solutions Ltd | Information & communication | 2022 |
| Optima Solutions (Cambs) Ltd | Professional & scientific | 2025 |
| Optimation (U.K.) Limited | Professional & scientific | 2023 |
| Optimize Management Ltd | Professional & scientific | 2023 |
| Options Autism (1) Limited | Education | 2000 |
| Options Autism (4) Limited | Real estate | 2006 |
| Options for Women Ltd | Financial services | 2000 |
| Opusone Limited | Real estate | 2025 |
| Oram Contracts Limited | Construction | 2014 |
| ORANGEPIPS42 Limited | Real estate | 2025 |