| Nozl Ltd | Manufacturing | 2021 |
| Nptx Ltd | Transport & logistics | 2023 |
| NRG Orthopaedic Services Limited | Health & social care | 2010 |
| NSA Associates Limited | Real estate | 2012 |
| NSC Motion Ltd | Information & communication | 2025 |
| NSM Technologies Limited | Information & communication | 2025 |
| NU Edge Works Ltd | Professional & scientific | 2025 |
| Nuko 74 Limited | Manufacturing | 2005 |
| Numenius Limited | Information & communication | 2024 |
| Nurse BEC Supplies Ltd | Wholesale & retail | 2023 |
| Nutranova Labs Ltd | Arts & recreation | 2025 |
| NVL Property Management Limited | Real estate | 2022 |
| NX Magazine Ltd | Information & communication | 2019 |
| NX Media & Marketing Limited | Manufacturing | 2024 |
| O & M Mechanical Services Ltd | Construction | 2021 |
| O a T Holdings Limited | Administrative & support | 2005 |
| O&C Orthodontics Ltd | Health & social care | 2016 |
| O'Connor & Sons Ltd | Construction | 2024 |
| O'Gara Property Management Limited | Households | 2018 |
| O.Boyce Solutions Ltd | Other services | 2022 |
| Oaks Escapes Ltd | Real estate | 2026 |
| OBC Cellar Tech Services Limited | Hospitality | 2016 |
| Obsidian Autoworks Limited | Wholesale & retail | 2023 |
| Oceania Property Investment Limited | Real estate | 2026 |
| Odelle Technology Limited | Information & communication | 2006 |
| Ofarrell IT Consulting Ltd | Information & communication | 2022 |
| OFF Book Collective Ltd | Arts & recreation | 2025 |
| Office and Home Furniture Ltd | Wholesale & retail | 2024 |
| Ogma CMO Ltd | Professional & scientific | 2026 |
| Old Beech Limited | Professional & scientific | 2011 |
| Old Chalford Estate Limited | Construction | 1996 |
| Old Street Films Limited | Information & communication | 1991 |
| Olebra Limited | Real estate | 2021 |
| Oliver Rowland Management Ltd | Financial services | 2021 |
| Olivia Gray Limited | Other services | 2024 |
| Ollie OWL Ltd | Education | 2012 |
| Olododo Consulting Ltd | Information & communication | 2025 |
| Oluaptransports Ltd | Transport & logistics | 2025 |
| Omnimed Cleaning Ltd | Administrative & support | 2026 |
| ON Your Terms Limited | Professional & scientific | 2018 |
| Onefiveseven Agency Ltd | Arts & recreation | 2026 |
| Oneson Properties Limited | Real estate | 2023 |
| Onestop Property Care Ltd | Energy | 2025 |
| Only Orthodontics Limited | Health & social care | 2023 |
| Onomatic Ltd | Information & communication | 2010 |
| Ontime Express Ltd | Transport & logistics | 2024 |
| Onxi Ltd | Professional & scientific | 2020 |
| Ooono UK Limited | Wholesale & retail | 2002 |
| Oovii Ltd | Manufacturing | 2026 |
| Opal Labs Limited | Information & communication | 2015 |
| Opal Locations Ltd | Professional & scientific | 2017 |
| Opera Della Luna | Arts & recreation | 1994 |
| OPP Limited | Information & communication | 2017 |
| OPS (Oxford) Ltd | Real estate | 2018 |
| Optima Training (UK) Limited | Administrative & support | 2003 |
| Optimise Psychology Limited | Education | 2025 |
| Optimum BD Ltd | Wholesale & retail | 2026 |
| Opus Leap Consulting Ltd | Wholesale & retail | 2025 |
| OR Components Ltd | Wholesale & retail | 2020 |
| Orange Yellow Limited | Professional & scientific | 2018 |
| Orchard Organic's Limited | Education | 1999 |
| Orchard Rooms Ltd | Other services | 2023 |
| Orchy Integration Solutions Limited | Professional & scientific | 2005 |
| Order of the Empire Limited | Manufacturing | 2020 |
| Oriel Innovation Services Limited | Administrative & support | 2023 |
| Oriel Investment Services Limited | Financial services | 2023 |
| Oriel Services Group Limited | Administrative & support | 2023 |
| Oriel Services Limited | Professional & scientific | 2019 |
| Oriental BU Trading Ltd | Wholesale & retail | 2007 |
| Oriental Food Catering Ltd | Hospitality | 2026 |
| Origin Family Health Birmingham Ltd | Health & social care | 2026 |
| Origin Family Health Ltd | Health & social care | 2026 |
| Orion Holdings (SE) LLP | | 2022 |
| Orion Medical Ltd | Extraterritorial | 2010 |
| Orion Medical Supplies Ltd | Wholesale & retail | 2012 |
| Orisoun Ltd | Administrative & support | 2017 |
| ORL Advisory Ltd | Professional & scientific | 2026 |
| Ormskirk Property Holdings Limited | Real estate | 2026 |
| Orpen Goods Ltd | Wholesale & retail | 2023 |
| Orthodontic Suite (Bexleyheath) Limited | Health & social care | 2023 |
| Ortus Property Group Ltd | Real estate | 2024 |
| Oréa Aesthetics Ltd | Health & social care | 2026 |
| Oréa Holdings Ltd | Real estate | 2024 |
| Oréa Salon Bicester Ltd | Other services | 2026 |
| Osta Ltd | Professional & scientific | 2024 |
| Othala Property Ltd | Real estate | 2015 |
| OUR World Events Limited | Arts & recreation | 2024 |
| Outrun Performance Ltd | Professional & scientific | 2026 |
| Outta the Smoke Ltd | Hospitality | 2020 |
| OWL and Starling Lodges Limited | Real estate | 2026 |
| OX Autos Limited | Wholesale & retail | 2015 |
| OX Building Design Ltd | Professional & scientific | 2019 |
| OX Luxe Ltd | Real estate | 2021 |
| OX Roofing Ltd | Construction | 2020 |
| OX-7 Foods Limited | Hospitality | 2024 |
| OX2P Ltd | Administrative & support | 2002 |
| Oxbis Ltd | Financial services | 2020 |
| Oxbourne Consulting Limited | Real estate | 2025 |
| Oxbridge Education International Limited | Education | 2018 |
| Oxbuild RS Limited | Construction | 2016 |