| Octopi Limited | Professional & scientific | 2003 |
| Odaseva UK Limited | Information & communication | 2021 |
| OE Estates Ltd | Real estate | 2017 |
| OEM Investments Limited | Professional & scientific | 2025 |
| OFF the Market Ltd | Information & communication | 2023 |
| Off-Piste Media Ltd | Information & communication | 2015 |
| Offbeat Orange Ltd | Transport & logistics | 2020 |
| Office Move London Limited | Transport & logistics | 2018 |
| Official Three Peaks Challenge Limited | Extraterritorial | 2023 |
| Offline Luxury Clothing Ltd | Wholesale & retail | 2025 |
| OG Sports Partnership Limited | Arts & recreation | 2021 |
| Oggc LLP | | 2017 |
| Ogma Communications Ltd | Information & communication | 2016 |
| Ograc Property Letting Ltd | Real estate | 2016 |
| OH Racing Group Ltd | Wholesale & retail | 2024 |
| OH Salvation Ltd | Professional & scientific | 2019 |
| OJI Consulting Limited | Professional & scientific | 2014 |
| OJS Consulting Services Ltd | Professional & scientific | 2021 |
| OKA Logistics Ltd | Transport & logistics | 2023 |
| OKI Investments Ltd | Construction | 2019 |
| Okja UK Limited | Wholesale & retail | 2023 |
| OL Estates Ltd | Real estate | 2019 |
| Old Coach House Developments Limited | Construction | 2016 |
| Old Ellens Holdings Ltd | Administrative & support | 2011 |
| Old Ellens Properties Ltd | Real estate | 2010 |
| Old Grinstead Management Company Limited | Professional & scientific | 2007 |
| Old Johnian Charity Limited(The) | Health & social care | 1974 |
| Old Pilgrim Court Management Company Limited | Households | 1996 |
| Oldhouse Lane Limited | Real estate | 2018 |
| OLI Construction Ltd | Construction | 2014 |
| Olive Leaf London Limited | Wholesale & retail | 2024 |
| Oliver James Homes Limited | Construction | 2012 |
| Oliver Parks Ltd | Professional & scientific | 2016 |
| Oliver Perez Gardens Ltd | Administrative & support | 2019 |
| Oliver Watts Ltd | Information & communication | 2023 |
| Ollie Hancock LLP | | 2009 |
| Ollie Thorn Consultancy Ltd | Professional & scientific | 2024 |
| Olsen Animal Trust | Other services | 2015 |
| Olvia Limited | Real estate | 2015 |
| Olympia Scaffolding Ltd | Construction | 2022 |
| Olympus Strength Ltd | Arts & recreation | 2024 |
| OM Blue Energy Ltd | Professional & scientific | 2017 |
| OM Hair Partners Ltd | Other services | 2024 |
| Ombudsman Association | Other services | 2019 |
| Omnatec Limited | Professional & scientific | 2022 |
| Omniforce Limited | Information & communication | 2024 |
| Omnis Equus Ltd | Agriculture & fishing | 2022 |
| ON the Fifth Day Ltd | Wholesale & retail | 2012 |
| One Element Group Ltd | Administrative & support | 2017 |
| One One Commercial Ltd | Real estate | 2025 |
| One to One (Original) Limited | Health & social care | 2019 |
| One Trinity Ltd | Households | 2023 |
| One Way Gardeners Limited | Professional & scientific | 2017 |
| One Way OR Another Limited | Administrative & support | 2013 |
| Onecool Air Conditioning Services Ltd | Energy | 2022 |
| Online Leisure Group Limited | Information & communication | 2005 |
| Online Lighting (Retail) Limited | Administrative & support | 2018 |
| Online Perspectives Limited | Information & communication | 2009 |
| ONO Ltd | Hospitality | 2025 |
| Onslow Construction Group Ltd | Construction | 2022 |
| Onslow Investments Limited | Real estate | 2017 |
| Onslow Village (Garages) Limited | Households | 1984 |
| Onslow Village (Powell Close) Limited | Households | 1984 |
| Onslow Village (Wilderness Court) Limited | Households | 1984 |
| Onyx S-Commerce Limited | Information & communication | 2024 |
| Oomoo Ltd | Other services | 2016 |
| Opal Group Holdings Limited | Financial services | 2014 |
| Open Energy Market Limited | Information & communication | 2012 |
| Open Lane Motors Limited | Wholesale & retail | 2026 |
| Openhouse Wellness Limited | Education | 2022 |
| Openml Limited | Information & communication | 2019 |
| Openplay Ltd | Arts & recreation | 2011 |
| Opensphere Ltd | Wholesale & retail | 2025 |
| Operate Surrey Limited | Education | 2022 |
| Opex Developments Limited | Construction | 2026 |
| Opia International (UK) Ltd | Administrative & support | 2013 |
| Opia Limited | Information & communication | 2006 |
| Opix 13 Limited | Information & communication | 2004 |
| Opos Creative Ltd | Information & communication | 2026 |
| Opportunity DA1 Limited | Financial services | 2022 |
| Opposite Peaks Ltd | Wholesale & retail | 2020 |
| Optima Medical Limited | Professional & scientific | 1999 |
| Optimaise Ltd | Information & communication | 2025 |
| Optimal Health Inspired Ltd | Health & social care | 2017 |
| Optimal People Limited | Administrative & support | 2019 |
| Optimatum Ltd | Manufacturing | 2023 |
| Optimise Aesthetics Limited | Health & social care | 2019 |
| Optimum Companies Limited | Professional & scientific | 2014 |
| Optimum Golf Technologies Limited | Wholesale & retail | 2010 |
| Optimum Structures Limited | Construction | 2011 |
| Opus OIL and GAS Limited | Mining & quarrying | 2016 |
| Orange Pizza Van Ltd | Hospitality | 2022 |
| Oras Clinic Limited | Health & social care | 2023 |
| Orasys Limited | Information & communication | 2010 |
| Orbis Communications Limited | Wholesale & retail | 2025 |
| Orbit Labs Ltd | Information & communication | 2024 |
| Orbittech Limited | Information & communication | 2015 |
| Orburn Limited | Professional & scientific | 2023 |
| Orchard Asset Management Limited | Real estate | 2002 |
| Orchard Cottage Interiors Limited | Other services | 1992 |