| One Moment in Time - LLP | | 2024 |
| One Motion Studio Ltd | Information & communication | 2026 |
| One Order Ventures Ltd | Wholesale & retail | 2025 |
| One Palliser Road Management Company Limited | Households | 1995 |
| One Parade Ltd | Real estate | 2020 |
| One SE8 Residents Company Limited | Households | 2001 |
| One Step by Step Limited | Health & social care | 2010 |
| One Step Flooring Ltd | Construction | 2020 |
| One Stop 247 Limited | Wholesale & retail | 2023 |
| One Stop Fleet Management Limited | Transport & logistics | 2024 |
| One Travel Solution LLP | | 2017 |
| One Wedding Fashions Limited | Wholesale & retail | 2015 |
| One Wholelistic Counselling Limited | Financial services | 2017 |
| One World Traders Ltd | Wholesale & retail | 2024 |
| ONE2NINE Limited | Households | 2000 |
| ONE4M Ltd | Professional & scientific | 2011 |
| Onecall Staff Benefits Limited | Administrative & support | 2016 |
| Onecoo Foods Ltd | Extraterritorial | 2020 |
| Onecoo Motors Limited | Administrative & support | 2020 |
| Onefix Plumbing & Building Solutions Ltd | Construction | 2023 |
| Oneflow Solutions Ltd | Wholesale & retail | 2026 |
| ONENINE5 Ltd | Manufacturing | 2018 |
| Onesec Incorporation Limited | Administrative & support | 2023 |
| Onestopsports Limited | Wholesale & retail | 2024 |
| Onetha's Care Services Limited | Extraterritorial | 2022 |
| Oneuengr Ltd | Information & communication | 2020 |
| Onevirtus Ltd | Real estate | 2020 |
| Ongar Cosmetic Practice Limited | Health & social care | 2014 |
| Ongar Road Management (Darlington Court) Limited | Households | 1985 |
| Onix Property Service Ltd | Extraterritorial | 2024 |
| Onkar Groups Ltd | Information & communication | 2022 |
| Online CAD Limited | Professional & scientific | 2011 |
| Online Company Trading Ltd | Other services | 2020 |
| Online Online Limited | Other services | 2016 |
| Online Studios Ltd | Arts & recreation | 2006 |
| Online Tradere Limited | Wholesale & retail | 2025 |
| Only Cells Ltd | Wholesale & retail | 2024 |
| Only Lofts and Extensions Limited | Construction | 2024 |
| Only One Management Limited | Professional & scientific | 2026 |
| Only Online Ltd | Wholesale & retail | 2024 |
| Onnline Solutions Limited | Wholesale & retail | 2019 |
| Onpoint Healthcare Group Ltd | Health & social care | 2023 |
| Onpoint Healthcare Training Ltd | Professional & scientific | 2018 |
| ONS Homes Limited | Real estate | 2025 |
| Onslow Avenue Mansions Residents Limited | Households | 1998 |
| Onslow Court (Worthing) RTM Company Limited | Households | 2007 |
| Ontrack Plumbing & Heating Ltd | Construction | 2026 |
| Onults Limited | Information & communication | 2011 |
| Onwards Construction Ltd | Construction | 2026 |
| Onyx Clothing Ltd | Wholesale & retail | 2026 |
| Onyx Media Ltd | Professional & scientific | 2017 |
| Ooey Gooey Ltd | Hospitality | 2026 |
| Oojageer Consulting Ltd | Information & communication | 2025 |
| Oonlyonee.Life Limited | Other services | 2025 |
| OOS Properties Limited | Real estate | 2021 |
| Opaay Ltd | Information & communication | 2022 |
| Opal Interiors Limited | Households | 2019 |
| Open Cube Ltd | Manufacturing | 2025 |
| Open House Arts Club Limited | Administrative & support | 2022 |
| Open MAT Productions Ltd | Information & communication | 2025 |
| Open Property Solutions Limited | Financial services | 2014 |
| Openair Medical Devices Limited | Wholesale & retail | 2020 |
| Openair Systems Limited | Information & communication | 2009 |
| Opendoors Medicare Ltd | Administrative & support | 2019 |
| Openlink Group Limited | Information & communication | 1992 |
| Openlink Software (UK) Limited | Information & communication | 2002 |
| Openlink.Com Limited | Information & communication | 2002 |
| Openmind Venture Ltd | Real estate | 2015 |
| Openminds Social Care Ltd | Health & social care | 2014 |
| Openware Computer Systems UK Ltd | Information & communication | 2020 |
| Operadora 2612 Ltd | Wholesale & retail | 2023 |
| Operum Tech Ltd | Extraterritorial | 2022 |
| Opes Knight Ltd | Financial services | 2012 |
| Opess Investments Limited | Financial services | 2025 |
| Opord Group Ltd | Administrative & support | 2023 |
| Opserv Ltd | Information & communication | 2024 |
| OPT Tech (UK) Limited | Wholesale & retail | 2014 |
| Opta Service Ltd | Construction | 2019 |
| Opti Europe Ltd | Information & communication | 2016 |
| Optilabs Limited | Manufacturing | 2019 |
| Optima Media Intl Ltd | Wholesale & retail | 2016 |
| Optima Planning Engineering Ltd | Professional & scientific | 2014 |
| Optimism Healthcare Support Limited | Health & social care | 2025 |
| Optimism Supported Housing | Real estate | 2019 |
| Optimized Neurotech Ltd | Wholesale & retail | 2019 |
| Optimum Care Plus Limited | Health & social care | 2021 |
| Optimum Drive Limited | Manufacturing | 2008 |
| Optimum Finance Ltd | Professional & scientific | 2012 |
| Optimum Heating Solutions Ltd | Construction | 2018 |
| Optimum Productions Ltd | Other services | 2016 |
| Optimum Specialised Homes Limited | Health & social care | 2008 |
| Options Cuisine Limited | Hospitality | 2022 |
| Options Estate Agents Ltd | Real estate | 2009 |
| Options IT Consultancy Limited | Information & communication | 2023 |
| Optomlocum Ltd | Health & social care | 2021 |
| Opton Telecom Limited | Information & communication | 2010 |
| Opulence London Limited | Construction | 2021 |
| Opulence Wealth Management UK Ltd | Financial services | 2017 |
| Opulent Aesthetics by Sadie Ltd | Other services | 2024 |
| Opulent Living Management & Rentals Ltd | Households | 2026 |