| One Corner Estates Ltd | Households | 2023 |
| One DAD at a Time Foundation | Information & communication | 2025 |
| One Nature Homes Ltd | Construction | 2020 |
| One Oak London Ltd | Health & social care | 2021 |
| One Pixel Push Ltd | Professional & scientific | 2025 |
| One Source Packaging Limited | Administrative & support | 2020 |
| One Stop Auto Service Limited | Wholesale & retail | 2010 |
| One Stop Bromley Ltd | Wholesale & retail | 2025 |
| One the OHR Ltd | Other services | 2023 |
| One Turpington Lane Freehold Limited | Real estate | 2019 |
| One World Commodities Limited | Administrative & support | 2013 |
| ONE2ONE Professional Coaching Limited | Education | 2003 |
| Onega Holdings Ltd | Administrative & support | 2024 |
| Onega Properties Ltd | Real estate | 2020 |
| Oneiione Limited | Administrative & support | 2016 |
| Ongar Road (Brentwood) Management Company Limited | Households | 2010 |
| Onibell Limited | Administrative & support | 2017 |
| Online Creations Entertainment Limited | Professional & scientific | 2025 |
| Only One Trading Ltd | Wholesale & retail | 2023 |
| Onroad Couriers Limited | Transport & logistics | 2019 |
| Onsiren Limited | Information & communication | 2026 |
| Onyx Consulting Limited | Professional & scientific | 2026 |
| OOH LA LAA Limited | Other services | 2008 |
| Opah Limited | Households | 2013 |
| Opal Forensics Limited | Professional & scientific | 2025 |
| Opensourcetester Limited | Information & communication | 2015 |
| Operational Advisory Ltd | Professional & scientific | 2026 |
| Opex Fashion Ltd | Wholesale & retail | 2010 |
| Ophthalmic Medical Consulting Ltd | Health & social care | 2019 |
| Opia Accountancy Ltd | Professional & scientific | 2024 |
| Opintels Solutions Ltd | Administrative & support | 2023 |
| Oppidan Apartments Management Limited | Households | 1999 |
| Oprent Limited | Information & communication | 2009 |
| Opslifeuk Limited | Information & communication | 2020 |
| Optimus Risk Solutions Limited | Financial services | 2026 |
| Optume Enterprise Limited | Wholesale & retail | 2021 |
| Orchard Green (Bromley) Management Limited | Households | 2013 |
| Orchard House (Bickley) Developments Ltd | Construction | 2019 |
| Orchard House (Bickley) Limited | Real estate | 2001 |
| Orchid Court (Bromley) RTM Company Limited | Households | 2013 |
| Orchid Fresh Homes Ltd | Other services | 2018 |
| Orient Unitech Co., Ltd | Manufacturing | 2012 |
| Origin District Ltd | Real estate | 2021 |
| Origin Wills Ltd | Professional & scientific | 2023 |
| Original Roofing and Construction Ltd | Construction | 2016 |
| Orion Star Industry Ltd | Extraterritorial | 2017 |
| Orla London Ltd | Extraterritorial | 2025 |
| Orpin (Forest Hill) Limited | Real estate | 1979 |
| Orpington Day Nursery Ltd | Education | 2020 |
| ORTHOM8 Ltd | Education | 2021 |
| Oryx Care Services Limited | Health & social care | 2023 |
| Os-Trilogy Ltd | Information & communication | 2025 |
| Os-Triology Ltd | Professional & scientific | 2025 |
| OSA Teknecilik Ltd | Extraterritorial | 2024 |
| Osaava Limited | Wholesale & retail | 2022 |
| Osborne House (Dagnall Park) RTM Company Ltd | Households | 2018 |
| Osbright & Company Limited | Administrative & support | 2014 |
| Oscar & Hugo Ltd | Real estate | 2019 |
| Oscar Close Management Company Limited | Households | 2013 |
| Oskar Driver Van Ltd | Transport & logistics | 2021 |
| Ospd Limited | Administrative & support | 2019 |
| OSS Time & Co Ltd | Wholesale & retail | 2026 |
| Ostc Consultants Limited | Professional & scientific | 2009 |
| Ostc Dubai Partnership Limited | Financial services | 2013 |
| Ostc Group Holdings Limited | Financial services | 2019 |
| Ostc Kondor LLP | | 2016 |
| OT Plus Limited | Administrative & support | 2025 |
| Othellis Limited | Real estate | 2024 |
| Otsaks Limited | Health & social care | 2021 |
| Otsaks Property Ltd | Real estate | 2022 |
| Otto EAT UK Limited | Hospitality | 2021 |
| Ottoway (Bond Issuer) UK PLC | Real estate | 2014 |
| Ottoway (City Portfolio) UK Limited | Real estate | 2014 |
| Ottoway (Sutton Bonington) UK Limited | Real estate | 2014 |
| Ottoway Accommodation Wallace Limited | Real estate | 2024 |
| Ottoway Accommodation Wallace Propco Limited | Real estate | 2025 |
| Ottoway Services Limited | Financial services | 2021 |
| Ottoway Wallace Services Limited | Real estate | 2025 |
| OU Property Rentals Ltd | Real estate | 2024 |
| Oudluxeroyale Ltd | Wholesale & retail | 2026 |
| Ourhouselondon Ltd | Construction | 2023 |
| OUT the Box Development Limited | Construction | 2022 |
| OUT the Box UD Ltd | Professional & scientific | 2020 |
| Outline Media Limited | Professional & scientific | 2008 |
| Outright 11 Limited | Construction | 2022 |
| Outside Group Ltd | Information & communication | 2026 |
| OUTSIDE2INSIDE Limited | Manufacturing | 2021 |
| Outsource-Sa Solutions Limited | Administrative & support | 2021 |
| Outstanding Care Homes Ltd | Health & social care | 2024 |
| Outwidedigital Ltd | Professional & scientific | 2025 |
| Overdale Management Company (Shortlands) Limited | Households | 1982 |
| Overdrive Limited | Professional & scientific | 2012 |
| Overseas Visas (UK) Limited | Real estate | 2007 |
| Oversite South Limited | Information & communication | 2022 |
| Owen Kent Limited | Arts & recreation | 2019 |
| Owlett Heating Ltd | Construction | 2020 |
| Own Your Runway- Skinz by Flare C.I.C. | Education | 2025 |
| OWO Ltd | Health & social care | 2015 |
| Oxbridge Studies Consult Ltd | Education | 2025 |
| Oxford & Bond Limited | Professional & scientific | 2020 |