| Oak Accountancy Ltd | Professional & scientific | 2024 |
| Oak House Acquisition Co Limited | Real estate | 2022 |
| Oak House Workspace Limited | Administrative & support | 2022 |
| Oak Labs Tech Limited | Information & communication | 2026 |
| Oak Meadow Limited | Agriculture & fishing | 1988 |
| Oak Technologies (Holdings) Ltd | Information & communication | 2023 |
| Oak Technologies Ltd | Information & communication | 2021 |
| Oakalls Physiotherapy Ltd | Health & social care | 2022 |
| Oakapple Developments Limited | Construction | 2021 |
| Oakapple Ltd | Real estate | 2018 |
| Oakdene Property Rentals Limited | Real estate | 2019 |
| Oakfield Ventures Limited | Administrative & support | 2024 |
| Oakhanger Consulting Services Limited | Information & communication | 2010 |
| Oakland Air Conditioning Limited | Professional & scientific | 2014 |
| Oakland Air Control Limited | Energy | 1993 |
| Oakland Estates (Holdings) Limited | Real estate | 2019 |
| Oakland Estates Limited | Construction | 2007 |
| Oakland Property Services Ltd. | Professional & scientific | 1997 |
| Oaklands Management (Ronkswood) Limited | Households | 1991 |
| Oakley Computer Training Ltd | Information & communication | 2002 |
| Oakmont Norfolk Holdings Limited | Financial services | 2026 |
| Oakridge Capital Finance Ltd | Financial services | 2026 |
| Oakrock Ltd | Administrative & support | 2025 |
| Oakstone Windows & Fascias Ltd | Construction | 2014 |
| Oakwood Development and Construction Ltd | Construction | 2017 |
| Oakwood Property Developers (UK) Ltd | Construction | 2006 |
| Oasis Alternative Provision Ltd | Education | 2024 |
| Oasis Cleaning Commercial Ltd | Administrative & support | 2023 |
| Oasis Cleaning SE Ltd | Administrative & support | 2025 |
| Oasis Cleaning Solutions Ltd | Administrative & support | 2022 |
| Oasis Cleaning South Ltd | Administrative & support | 2025 |
| Oasis Palm Tree Ltd | Information & communication | 2013 |
| Obex Holdings Ltd | Financial services | 2021 |
| Obex Protection Ltd | Manufacturing | 2014 |
| Objects Simple Limited | Information & communication | 2019 |
| OBM Media Ltd | Professional & scientific | 2022 |
| OC Origami Limited | Professional & scientific | 2023 |
| Occon Limited | Financial services | 2008 |
| Ocean Blue Digital Print Ltd | Manufacturing | 2020 |
| Ocean Blue Graphics Limited | Arts & recreation | 2014 |
| Ochre Energy Limited | Administrative & support | 2025 |
| Ockeridge Properties Ltd | Real estate | 2023 |
| Octagon I/O Ltd | Information & communication | 2014 |
| OCV Automotive Ltd | Wholesale & retail | 2017 |
| OCV Care Ltd | Health & social care | 2020 |
| Oddkin Coffee Limited | Hospitality | 2023 |
| Oddstepperz Limited | Arts & recreation | 2019 |
| ODJ Logistics Limited | Transport & logistics | 2015 |
| OFF the Bike Ltd | Wholesale & retail | 2025 |
| Offbeat Escapes Limited | Administrative & support | 2013 |
| Ofiaz Limited | Professional & scientific | 2024 |
| OG Systems Ltd | Construction | 2021 |
| Oguzboyu Limited | Hospitality | 2024 |
| OIL Spill Service Company Limited | Wholesale & retail | 2017 |
| Oishee Foods Ltd | Hospitality | 2026 |
| Ojah Properties Limited | Real estate | 2003 |
| Ojas Health Intelligence Ltd | Health & social care | 2026 |
| Okavango Skimmer Safaris Ltd | Administrative & support | 2026 |
| Okowi Partners Limited | Health & social care | 2025 |
| OLA Swims Ltd | Manufacturing | 2025 |
| Olavo Ltd | Professional & scientific | 2025 |
| Old Chantry School (Martley) Management Company Limited | Households | 1991 |
| Old Eign (Common Areas) | Real estate | 2024 |
| Old Elizabethans Cricket Club Ltd | Arts & recreation | 2016 |
| Old Hall Properties Limited | Professional & scientific | 2015 |
| Old Library (Lichfield) Management Company Limited | Households | 2025 |
| Old Q Head Halesowen Ltd | Hospitality | 2024 |
| Old Springbank Limited | Households | 2006 |
| OLE Homes Ltd | Real estate | 2024 |
| Oliver Dakers Blacksmiths Limited | Manufacturing | 2012 |
| Oliver's Army Limited | Professional & scientific | 1999 |
| Olivia Thomas Estates Ltd | Other services | 2024 |
| Olpro Ltd | Wholesale & retail | 2011 |
| OM Bidco Limited | Information & communication | 2014 |
| Omatsonleo Consults Limited | Wholesale & retail | 2025 |
| Ombersley Trading Company Ltd | Construction | 2017 |
| Omega HR Limited | Administrative & support | 2017 |
| Omnar Lenses Limited | Manufacturing | 2023 |
| Omni PC Limited | Wholesale & retail | 2003 |
| Omnia 7 Ltd | Other services | 2022 |
| Omnilyt Ltd | Wholesale & retail | 2024 |
| Omnirx Limited | Information & communication | 2026 |
| Omnixa Group Ltd | Construction | 2022 |
| ON a Role Limited | Arts & recreation | 2008 |
| ON Call Healthcare Services & Training Ltd | Health & social care | 2025 |
| ON Court Apps Ltd | Information & communication | 2025 |
| Oncall Healthcare Limited | Health & social care | 2019 |
| One Acre Wood Ltd | Hospitality | 2026 |
| One Avenue Recruitment Limited | Administrative & support | 2007 |
| One Duckling Ltd | Real estate | 2024 |
| One Intention Products Ltd | Wholesale & retail | 2022 |
| One Primary Care Limited | Extraterritorial | 2026 |
| One Step Worcester Limited | Professional & scientific | 2022 |
| One WOR1D Ltd | Wholesale & retail | 2021 |
| ONE10 Creative Ltd | Professional & scientific | 2011 |
| Oneskee Limited | Wholesale & retail | 2014 |
| Onevision Europe Limited | Wholesale & retail | 2013 |
| Online Rental Property Limited | Real estate | 2019 |
| Onside Independent Advocacy | Health & social care | 2003 |
| Onthepipe Ltd | Arts & recreation | 2024 |