| Office Klerk Ltd | Information & communication | 2024 |
| Officer Theatricals Limited | Administrative & support | 2015 |
| Officer UK Tour Ltd | Arts & recreation | 2023 |
| OG Developments Limited | Real estate | 2021 |
| Ogmi Ltd | Manufacturing | 2017 |
| Ogoja Limited | Arts & recreation | 2012 |
| Oj'Ssecurityltd Ltd | Administrative & support | 2026 |
| OK Flowers Ltd | Wholesale & retail | 2019 |
| OLA ALL Stars Limited | Arts & recreation | 2025 |
| Old Falcon House Ltd | Wholesale & retail | 2025 |
| Old School Terrace Residents Company Limited | Households | 1994 |
| Oldfields 2 Ltd | Financial services | 2024 |
| Oldfields 3 Ltd | Financial services | 2024 |
| Oldfields 4 Ltd | Financial services | 2024 |
| Oldfields 5 Ltd | Financial services | 2024 |
| Olicity Support Services Limited | Health & social care | 2016 |
| Olif Ltd | Wholesale & retail | 2010 |
| Oliva Maram Limited | Health & social care | 2025 |
| Olive Clarke Limited | Information & communication | 2022 |
| Olive Tree Praise Ministries | Other services | 2010 |
| Oliver Francis Development Limited | Real estate | 2016 |
| Olives Peri Peri Chicken & Pizza Ltd | Hospitality | 2017 |
| Oloco Ltd | Information & communication | 2014 |
| Olympus Building Ltd | Construction | 2024 |
| OM Cars Ltd | Transport & logistics | 2022 |
| Omega One Consultancy Ltd | Administrative & support | 2019 |
| Omniora Limited | Wholesale & retail | 2025 |
| Omoghan Consultancy Limited | Education | 2013 |
| Omsiva Limited | Administrative & support | 2024 |
| OMW London Ltd | Construction | 2019 |
| ON Point Tracking Ltd | Wholesale & retail | 2019 |
| ON the Marathon Ltd | Wholesale & retail | 2026 |
| ON Top Properties Ltd | Real estate | 2023 |
| One Aurora Limited | Wholesale & retail | 2022 |
| One Day UK Tour Ltd | Arts & recreation | 2025 |
| One Day West END Ltd | Arts & recreation | 2023 |
| One Day West END TOO Ltd | Arts & recreation | 2025 |
| One in a Cillian Ltd | Information & communication | 2024 |
| One More Consulting Limited | Professional & scientific | 2018 |
| One OFF Developments 1 Ltd | Construction | 2021 |
| One Song Scene Limited | Information & communication | 2010 |
| One Step Beyond Flooring Ltd | Construction | 2026 |
| One Stop Party Distribution Limited | Wholesale & retail | 2022 |
| One Time Boilers Ltd | Construction | 2023 |
| One Touch Plumbing and Heating Limited | Construction | 2019 |
| One Venture Hill Capital Limited | Financial services | 2025 |
| One Waste London Limited | Water & waste | 2019 |
| One Waste Ltd | Water & waste | 2021 |
| Oneapps Consulting Limited | Information & communication | 2017 |
| Oneapps Enterprise Technologies Ltd | Information & communication | 2007 |
| Onebuild Group Ltd | Other services | 2023 |
| Onebuild Projects Limited | Professional & scientific | 2025 |
| Onepotato Ltd | Professional & scientific | 2026 |
| Onevoice (UK) Limited | Information & communication | 2013 |
| Ongoren Limited | Hospitality | 2024 |
| Ongraph Technologies Limited | Information & communication | 2007 |
| Onion and Bunion Ltd | Households | 2014 |
| Onlead UK Ltd | Real estate | 2021 |
| Online Accounts & Tax Services Ltd | Professional & scientific | 2021 |
| Online Advertising Expert Ltd | Professional & scientific | 2024 |
| Online Bazzar UK Limited | Wholesale & retail | 2022 |
| Onslow Motors Limited | Wholesale & retail | 2005 |
| Ontopmanagement Ltd | Information & communication | 2025 |
| Onyx Decs Ltd | Construction | 2026 |
| Onyx PAK Trader Ltd | Transport & logistics | 2016 |
| OP Horizon Limited | Construction | 2026 |
| Opal Housing Ltd | Health & social care | 2021 |
| Opal Property Solutions Private Limited | Real estate | 2021 |
| Open Book Theatre Limited | Arts & recreation | 2015 |
| Opengear Limited | Wholesale & retail | 2010 |
| Opensky Property Ltd | Real estate | 2026 |
| Ophel Capital Ltd | Professional & scientific | 2025 |
| Opportunity Central Consultancy Ltd | Professional & scientific | 2019 |
| Optima Financial Limited | Extraterritorial | 2000 |
| Optimeyes EE Investments Limited | Professional & scientific | 2019 |
| Optimeyes Energy Limited | Professional & scientific | 2017 |
| Optimeyes Investments Limited | Professional & scientific | 2019 |
| Optimised Life Ltd | Information & communication | 1998 |
| Optimize Plumbing & Heating Ltd. | Construction | 2016 |
| Optimum Refurb Ltd | Construction | 2020 |
| Optimyze Finance LLP | | 2023 |
| Optionomical Limited | Real estate | 2007 |
| Options Confidential Waste Services Limited | Water & waste | 2005 |
| Optirisk Systems Limited | Information & communication | 1971 |
| ORANGE95 Limited | Information & communication | 2025 |
| Orbit Accounting Services Ltd | Professional & scientific | 2019 |
| Orbit Star Studios Ltd | Information & communication | 2025 |
| Orbitin Limited | Information & communication | 2026 |
| Orca Logic Limited | Information & communication | 1997 |
| Orchard Hill College | Education | 2011 |
| Orchard Hill College Academy Trust | Education | 2013 |
| Orchard MSP Group Limited | Wholesale & retail | 2025 |
| Orchard Tradings Limited | Wholesale & retail | 2024 |
| Organic ME Group Ltd | Wholesale & retail | 2017 |
| Oriental Health Oasis Ltd | Health & social care | 2026 |
| Orimox Ltd | Professional & scientific | 2019 |
| Orinda Ltd | Households | 2025 |
| Orla Contracts Limited | Construction | 2003 |
| Ormsby (Freehold) Limited | Real estate | 1989 |
| Ornex AI Limited | Information & communication | 2026 |