| O'Leary Legacy Holdings Ltd | Real estate | 2024 |
| O'Leary Legacy Travel Ltd | Administrative & support | 2026 |
| O'Lumi Studio Ltd | Information & communication | 2025 |
| O'Neil Investments Limited | Real estate | 2018 |
| O-Zone Ltd. | Extraterritorial | 1998 |
| O.E.G. Rigging Services Limited | Construction | 2017 |
| O.S Food Ventures Ltd | Hospitality | 2020 |
| O2C LAB Community Ltd | Administrative & support | 2023 |
| O2C LAB Events Ltd | Administrative & support | 2023 |
| O2C LAB Training & Performance Ltd | Administrative & support | 2023 |
| OAE Ultimate Limited | Health & social care | 2018 |
| Oagpa CIC | Education | 2022 |
| Oak and IVY Estates Ltd | Real estate | 2024 |
| Oak Croft Developments Limited | Administrative & support | 2007 |
| Oak Farm Interiors Ltd | Manufacturing | 2022 |
| Oak Green Services Ltd | Professional & scientific | 2008 |
| Oakfield Financial Management Limited | Professional & scientific | 2021 |
| Oakhall Court Management Limited | Construction | 2014 |
| Oakley Jane Limited | Professional & scientific | 2016 |
| Oakley Meadows Management Limited | Real estate | 2021 |
| Oakminster Holdings Limited | Financial services | 2026 |
| Oakminster Homes Limited | Construction | 2006 |
| Oakridge Building Services Ltd | Construction | 2026 |
| Oakwood Wellbeing Limited | Health & social care | 2016 |
| Oases Accounting Solutions Ltd | Professional & scientific | 2018 |
| Obsequio Consulting Ltd | Professional & scientific | 2025 |
| Obsessive Digital Ltd | Professional & scientific | 2018 |
| OC Engineering Surveying Ltd | Professional & scientific | 2026 |
| Occulus Limited | Construction | 2025 |
| Occupational Balance Ltd | Health & social care | 2024 |
| Ocean Image Marine Services Ltd | Transport & logistics | 2018 |
| Ocean Mount Zion Properties Ltd | Real estate | 2025 |
| Ocelot Solutions Ltd | Information & communication | 2001 |
| OCM EPF Specialist Lending Ltd | Financial services | 2017 |
| Ocssuk Ltd | Administrative & support | 2026 |
| ODD JOB SAM Ltd | Construction | 2025 |
| Oddworks Interactive Ltd | Information & communication | 2012 |
| Odomali Structures Ltd | Financial services | 2021 |
| Odpatrycjaszajna Ltd | Administrative & support | 2023 |
| Odrl Recruitment Solutions Ltd | Extraterritorial | 2020 |
| OFF Site Construction Interiors Limited | Construction | 2008 |
| Off-Road and Overland Adventures Ltd | Arts & recreation | 2021 |
| Offbeat Marketing Ltd | Professional & scientific | 2021 |
| Office Ergonomics Limited | Wholesale & retail | 2006 |
| Office Innovation Online Ltd | Information & communication | 2010 |
| Office IQ Ltd | Construction | 2025 |
| Offshore Danny Ltd | Manufacturing | 2021 |
| Ofinity Ltd | Wholesale & retail | 2021 |
| OGA Ltd | Professional & scientific | 2023 |
| Oggy V Plumbing Ltd | Construction | 2015 |
| OGR Ltd | Real estate | 2021 |
| Okflo Limited | Real estate | 2025 |
| Olayinka Properties Ltd | Real estate | 2024 |
| Olcc (Oxon) Ltd | Construction | 2025 |
| Old Bryers Close Management Company Limited | Households | 2005 |
| Old Chapel Restaurant Limited | Hospitality | 2026 |
| Old Henley Brewery (Freehold) Ltd | Households | 2009 |
| Old Manor Ickford Ltd | Hospitality | 2025 |
| Old Orchard (Iver) Management Limited | Households | 2005 |
| Old Oxford Investment Corporation Ltd | Real estate | 2010 |
| Oliver Lacey Limited | Information & communication | 2020 |
| Oliver's Plumbing and Heating Ltd | Construction | 2023 |
| Olivers Estates Ltd | Real estate | 2023 |
| Ollo Limited | Information & communication | 2004 |
| Oluwatoni Care Solutions Limited | Health & social care | 2014 |
| Olwyn Homes Limited | Real estate | 2024 |
| OM SAI Enterprise (Wendover) Limited | Wholesale & retail | 2017 |
| OM Shakthi Homes Ltd | Real estate | 2024 |
| OM Shakthi Properties Ltd | Real estate | 2024 |
| OM Shakthi Softech Ltd | Information & communication | 2017 |
| Omaka Group Ltd | Hospitality | 2020 |
| Omcv Management LLP | | 2025 |
| Omega Glass and Glazing Ltd | Construction | 2026 |
| Omega Health Management Limited | Health & social care | 2015 |
| Omega Healthcare Limited | Health & social care | 2009 |
| Omel Media Ltd | Manufacturing | 2025 |
| OMG Streetfood Ltd | Hospitality | 2019 |
| Omkaar Properties Ltd | Construction | 2024 |
| Omni Project Solutions Limited | Wholesale & retail | 2010 |
| Omnicloud Limited | Information & communication | 2014 |
| Omnikote Limited | Other services | 1999 |
| Omnis Consulting Limited | Professional & scientific | 2002 |
| Omomet Care Limited | Health & social care | 2016 |
| ON Call Healthcare Solutions Limited | Health & social care | 2013 |
| ON YER Bike Limited | Wholesale & retail | 1997 |
| Onaffiliate Agency Limited | Information & communication | 2022 |
| One Earth 740 SPV Limited | Financial services | 2023 |
| One Earth Solar Farm Limited | Energy | 2020 |
| One Family Properties Limited | Real estate | 2023 |
| One Momentum Transport Solutions Ltd | Transport & logistics | 2026 |
| One Planet Solar Farm Limited | Energy | 2019 |
| One Soft Limited | Administrative & support | 2018 |
| One Source Tech Ltd | Administrative & support | 2024 |
| One Stop Global Mart Ltd | Wholesale & retail | 2024 |
| One Stop SOL Ltd | Information & communication | 2025 |
| One Tree Interior Ltd | Manufacturing | 2017 |
| One Ummah Aylesbury Ltd | Hospitality | 2023 |
| One Universal Window Ltd | Wholesale & retail | 2023 |
| Oneil Property Solutions Ltd | Real estate | 2024 |
| Online Accounts Limited | Professional & scientific | 2011 |