| OK Style Group Limited | Construction | 2025 |
| OK4FREEKICK Football & Sports Management Ltd | Manufacturing | 2023 |
| Okam Property Ltd | Real estate | 2022 |
| Okwute Properties Ltd | Real estate | 2023 |
| Olabuy Ltd | Wholesale & retail | 2022 |
| Olad Heating and GAS Ltd | Construction | 2023 |
| Olad Limited | Health & social care | 2019 |
| Olaoluwa Foods Ltd | Wholesale & retail | 2025 |
| Olasint Ltd | Transport & logistics | 2022 |
| Olayinka Fambegbe Social Care Services Limited | Wholesale & retail | 2020 |
| Olaz Konsult Limited | Administrative & support | 2020 |
| OLC (Europe) Limited | Professional & scientific | 2002 |
| OLC College Limited | Education | 2013 |
| Old Cheadle Bar Limited | Hospitality | 2021 |
| Old Cross Pharmacy Limited | Wholesale & retail | 2010 |
| Old Holts Turf Ltd | Extraterritorial | 2012 |
| Old Oak Equestrian Ltd | Wholesale & retail | 2026 |
| Old River Group Ltd | Financial services | 2020 |
| Old Silk Route Ltd | Wholesale & retail | 2022 |
| Oldbridge Healthcare Limited | Health & social care | 2017 |
| Oldfield Bathrooms and Kitchens Limited | Construction | 2005 |
| Oldfield Road Management Company Limited | Real estate | 2026 |
| Oldham Lizzy Ltd | Hospitality | 2020 |
| Oldstead Elite Property Limited | Real estate | 2025 |
| Olexsys Limited | Professional & scientific | 2010 |
| Olgeno Limited | Construction | 2021 |
| Olia Decorating Ltd | Construction | 2021 |
| Olis Estates Ltd | Real estate | 2018 |
| Olits Services Limited | Transport & logistics | 2017 |
| Olive Educational Trust | Education | 2021 |
| Olive Family Practice Limited | Health & social care | 2010 |
| Olive Green Estates Ltd | Real estate | 2022 |
| Olive Tree Invest Ltd | Financial services | 2018 |
| Olive Veterinary Care Limited | Professional & scientific | 2021 |
| Oliver Ashworth (Holdings) Limited | Wholesale & retail | 2014 |
| Oliver Ashworth Limited | Wholesale & retail | 2014 |
| Oliver David Williams Consulting Limited | Information & communication | 2020 |
| Oliver Thomas Kitchens Ltd | Other services | 2020 |
| Oliver Tuck Limited | Professional & scientific | 2023 |
| Oliver Ward Holdings Ltd | Financial services | 2023 |
| Oliver Wotherspoon Ltd | Arts & recreation | 2009 |
| OLIVERSKITCHEN15 Limited | Hospitality | 2026 |
| Olivia's Garden Bar Ltd | Hospitality | 2026 |
| Olivias Properties Group Ltd | Real estate | 2024 |
| Oliyn World Limited | Wholesale & retail | 2025 |
| Ollerton Consulting Ltd | Construction | 2020 |
| Ollyrealcreations Limited | Hospitality | 2025 |
| Ollys Auto Repairs Ltd | Wholesale & retail | 2025 |
| OLM Property Group Limited | Real estate | 2026 |
| Olszak & SON Holding Ltd | Financial services | 2026 |
| Oluwa Ltd | Health & social care | 2020 |
| Olympus Crew Limited | Other services | 2018 |
| Olympus Express Ltd | Transport & logistics | 2023 |
| Olympus Fish and Chip Restaurant Limited | Hospitality | 2008 |
| Olyness Ltd | Education | 2006 |
| OM Cafe Ltd | Wholesale & retail | 2025 |
| OM Northwest Ltd | Wholesale & retail | 2026 |
| Omands Global Properties Limited | Real estate | 2022 |
| Omands Global Resources Limited | Wholesale & retail | 2023 |
| Omar Hraib Ltd | Professional & scientific | 2017 |
| Omari Group GB Ltd | Wholesale & retail | 2024 |
| Omark-A Property Ltd | Real estate | 2025 |
| Omarko Technologies Limited | Information & communication | 2026 |
| Omber Nationwide Ltd | Wholesale & retail | 2025 |
| Omega 247 Support Ltd | Health & social care | 2021 |
| Omega 64 Ltd | Wholesale & retail | 2026 |
| Omega Afro Caribbean Superstore Ltd | Wholesale & retail | 2024 |
| Omega Analytics Ltd | Professional & scientific | 2023 |
| Omega Commercials Ltd | Manufacturing | 2003 |
| Omega Devices UK Ltd | Wholesale & retail | 2025 |
| Omega Joinery Ltd | Construction | 2018 |
| Omega LED Supplies Ltd | Manufacturing | 2022 |
| Omichael IT Service Ltd | Information & communication | 2016 |
| Omid & Co Ltd | Professional & scientific | 2015 |
| OMK Customs Ltd | Wholesale & retail | 2022 |
| OMK Group Holdings Limited | Financial services | 2026 |
| Omni Analysis Ltd | Information & communication | 2018 |
| Omnia Information Management Ltd | Information & communication | 2018 |
| Omnia Ventures Ltd | Wholesale & retail | 2012 |
| Omnicore Mart Ltd | Wholesale & retail | 2025 |
| Omnicore Ventures Ltd | Wholesale & retail | 2025 |
| Omnific Services Ltd | Other services | 2024 |
| Omnific Traders Ltd | Wholesale & retail | 2026 |
| Omnis Vascular Limited | Health & social care | 2015 |
| Omnityx Limited | Professional & scientific | 2025 |
| Omol Properties Ltd | Professional & scientific | 2024 |
| Omranico Ltd | Construction | 2025 |
| OMT Services Limited | Professional & scientific | 2023 |
| ON Learning Ltd | Education | 1999 |
| ON Point Silhouette Ltd | Wholesale & retail | 2026 |
| ON SET CNC Ltd | Manufacturing | 2026 |
| ON Target Sales & Consultancy Limited | Information & communication | 2023 |
| ON Time Call Limited | Information & communication | 2026 |
| ON Time Courier Services Ltd Limited | Wholesale & retail | 2025 |
| ON to a Winner UK Limited | Arts & recreation | 2020 |
| ON Top Elite Coaching Limited | Education | 2026 |
| ON Web Consulting Ltd | Information & communication | 2011 |
| ON Your Own Terms Clothing Ltd | Wholesale & retail | 2024 |
| Onahi Ltd | Health & social care | 2023 |
| Onak Limited | Real estate | 2022 |