| O3 Host Limited | Information & communication | 2024 |
| Oak Construction South Limited | Construction | 2024 |
| Oak Hill (Poole) Limited | Households | 2017 |
| Oak Innovation Limited | Information & communication | 1987 |
| Oak Mews Management Company Limited | Households | 1990 |
| Oak Mount Care Home Limited | Health & social care | 2012 |
| Oak Tree (Poole) Management Company Limited | Households | 2004 |
| Oak Valley Plastering Ltd | Construction | 2026 |
| Oak Wellness Ltd | Professional & scientific | 2025 |
| Oakcourt Limited | Construction | 1995 |
| Oakdale Garage (Poole) Limited | Wholesale & retail | 2004 |
| Oakenshield Group Ltd | Professional & scientific | 2026 |
| Oakfields&Groups Limited | Information & communication | 2025 |
| Oakhouse Property Development Limited | Construction | 2012 |
| Oakhurst Court RTM Company Limited | Real estate | 2010 |
| Oakley Construction Ltd | Construction | 2023 |
| Oakley Leppington Property Ltd | Real estate | 2025 |
| Oakline Consultants Ltd | Professional & scientific | 2026 |
| Oakpark Security Solutions Ltd | Construction | 2021 |
| Oakstone International Ltd | Administrative & support | 2023 |
| Oakstone Limited | Administrative & support | 2000 |
| Oakvale Health Care Ltd | Health & social care | 2024 |
| Oakwood Business Consulting Ltd | Professional & scientific | 2021 |
| Oakwood Court (Bisley) Management Company Limited | Households | 1984 |
| Oarno Limited | Wholesale & retail | 2025 |
| Oasis Mobile Studio Limited | Wholesale & retail | 2025 |
| Oasis One Bournemouth Limited | Real estate | 2019 |
| Oasis One Management Limited | Households | 2013 |
| Oasis Plant Hire Limited | Real estate | 1986 |
| Oasis South Ltd | Wholesale & retail | 2013 |
| Oastcrest Limited | Real estate | 1984 |
| Oathelabel Ltd | Wholesale & retail | 2026 |
| Oatlands Court (Weybridge) Management Company Limited | Households | 2017 |
| OB Autotech Ltd | Wholesale & retail | 2025 |
| OBC Blade and Composite Repairs Limited | Manufacturing | 2026 |
| Objectable Ltd | Information & communication | 2011 |
| Obki Holdings Limited | Financial services | 2020 |
| Obki Productions Limited | Information & communication | 2019 |
| Obsideo Technologies Limited | Professional & scientific | 2000 |
| Obsidianweb Limited | Information & communication | 2014 |
| Obsolete Parts Ltd | Wholesale & retail | 2019 |
| OCCUPATION4LIFE Ltd | Education | 2020 |
| Ocean & Boardroom Ltd | Professional & scientific | 2017 |
| Ocean Automotive (Bodyshops) Limited | Wholesale & retail | 2015 |
| Ocean Automotive (Holdings) Limited | Professional & scientific | 2015 |
| Ocean Automotive (Swedish) Limited | Wholesale & retail | 2015 |
| Ocean Automotive Limited | Wholesale & retail | 2000 |
| Ocean Blue Developments Ltd | Construction | 2001 |
| Ocean Breeze Publishing Ltd | Information & communication | 2025 |
| Ocean Chem Ltd | Wholesale & retail | 2025 |
| Ocean Communications Ltd | Information & communication | 2018 |
| Ocean Digital Ltd | Information & communication | 2013 |
| Ocean Hare Ltd | Arts & recreation | 2018 |
| Ocean Lovers United Ltd | Wholesale & retail | 2020 |
| Ocean Performance Limited | Wholesale & retail | 2024 |
| Ocean Property Management Ltd | Real estate | 2026 |
| Ocean Reef Property Investment Limited | Real estate | 2026 |
| Ocean Ripples Ltd | Wholesale & retail | 2023 |
| Ocean Sands Limited | Hospitality | 2007 |
| Ocean Solutions Limited | Health & social care | 2025 |
| Oceanfront Construction Ltd | Construction | 2024 |
| Ocelot Accounting Ltd | Professional & scientific | 2019 |
| Ocelot Group Limited | Financial services | 2022 |
| OCF Investments Ltd | Real estate | 2025 |
| Ocgm Ltd | Other services | 2016 |
| Ockerman Agritech Limited | Information & communication | 2025 |
| OCKHAM1285 Limited | Professional & scientific | 2018 |
| Ockhaminvest Limited | Professional & scientific | 2020 |
| OCS Flats 9 & 10 Limited | Households | 2000 |
| Octave Property Ltd | Real estate | 2021 |
| Octavian Landscape & Construction Ltd | Construction | 2022 |
| Octochef Ltd | Real estate | 2018 |
| Octopress Printers Limited | Manufacturing | 2002 |
| OD Automation Innovation Ltd | Manufacturing | 2019 |
| Odoss Ltd | Wholesale & retail | 2021 |
| Ofan Aviation Limited | Manufacturing | 2025 |
| OFF Grid Solutions (UK) Limited | Information & communication | 2025 |
| OFF Key Design Ltd | Professional & scientific | 2025 |
| OFF the Grid Club Limited | Arts & recreation | 2025 |
| Offaly Good Dog Treat Company Ltd | Manufacturing | 2025 |
| Offer Holdings Limited | Real estate | 2019 |
| Offshore Marine Security Limited | Professional & scientific | 2018 |
| Offthetongue Ltd. | Manufacturing | 2019 |
| OG Bespoke Limited | Wholesale & retail | 2012 |
| Oguz Ltd | Professional & scientific | 2025 |
| Oh-Tech Limited | Information & communication | 2016 |
| OHANA25 Ltd | Hospitality | 2025 |
| OHF Carpentry&Joinery Ltd | Manufacturing | 2025 |
| OI Global Partners Limited | Extraterritorial | 2023 |
| Oidia Media Ltd | Wholesale & retail | 2021 |
| Oikoi Heritage Ltd | Professional & scientific | 2025 |
| Oikos Home Limited | Construction | 2025 |
| Ojmg Holdings Limited | Financial services | 2026 |
| OJN 97 Limited | Construction | 2021 |
| Ojo's Catering Ltd | Hospitality | 2023 |
| Ojo’S Street Kitchen Ltd | Hospitality | 2026 |
| Ojtb Group Limited | Construction | 2026 |
| OKR LLP | | 2009 |
| Okra Design Ltd | Information & communication | 2020 |
| Old & Campbell Limited | Wholesale & retail | 1918 |