| Novakal Limited | Financial services | 2018 |
| Novum Global Limited | Administrative & support | 2014 |
| Nrpc Consulting Ltd | Professional & scientific | 2019 |
| NS Signature Homes Ltd | Real estate | 2024 |
| Ntapers Brockley Limited | Other services | 2022 |
| Nu-Look Ltd. | Administrative & support | 2011 |
| Number 9 the Nursery Limited | Education | 2014 |
| Numerica Consultants Limited | Professional & scientific | 2014 |
| Nuria Catering Ltd | Hospitality | 2025 |
| NY Properties London Ltd | Real estate | 2025 |
| Nyky Ltd | Real estate | 2024 |
| Nzeru Investments Limited | Wholesale & retail | 2024 |
| O Holloway Consulting Limited | Information & communication | 2018 |
| O Imperio Brazilian Market Ltd | Wholesale & retail | 2023 |
| O Town Music Ltd | Information & communication | 2018 |
| O&E Construction Ltd | Construction | 2022 |
| O'Connell Project Management Limited | Administrative & support | 2012 |
| O'Connor Plant Hire Limited | Construction | 2015 |
| O'Finn Building Services Limited | Construction | 2015 |
| O'Shea DO Carmo Ltd | Professional & scientific | 2026 |
| O.K. Catering Limited | Hospitality | 2000 |
| O.P Food Limited | Hospitality | 2026 |
| Oak Cleaning Limited | Administrative & support | 2020 |
| Oakbrook Manor Limited | Real estate | 2005 |
| Oakfield House Care Limited | Health & social care | 2018 |
| Oakhouse Prime Management Ltd | Administrative & support | 2018 |
| Oakmere Law Limited | Administrative & support | 2017 |
| Oakmere Property Ltd | Real estate | 2018 |
| Oaks Management (Surrey) Limited | Real estate | 2019 |
| Oakwood Building Contractors Holdings Limited | Financial services | 2020 |
| Oakwood Group Holdings Limited | Construction | 2019 |
| Oasis Private Limited | Wholesale & retail | 2019 |
| Obsparkles Cleaning Solution Ltd | Administrative & support | 2025 |
| OC Orthopaedics Limited | Health & social care | 2024 |
| Occhi DA Volpe Ltd | Hospitality | 2026 |
| Ocean Blue Cleaning Ltd | Extraterritorial | 2024 |
| Oceanvisual Limited | Extraterritorial | 2006 |
| Oddly Events Ltd. | Arts & recreation | 2023 |
| Offgrid Holdings Ltd | Manufacturing | 2023 |
| OFL Brickwork Limited | Construction | 2022 |
| Ohana Fitness Ltd | Arts & recreation | 2020 |
| Okktopus Ltd | Construction | 2025 |
| Oklo World Limited | Information & communication | 2023 |
| OKM Clottey Ventures Ltd | Education | 2025 |
| Olaluxury Ltd | Wholesale & retail | 2025 |
| Old Kabul Ltd | Hospitality | 2019 |
| Old Whitgiftian Trustees Limited | Administrative & support | 1944 |
| Oliett Ltd | Wholesale & retail | 2025 |
| Olive & Pickle Catering Limited | Hospitality | 2021 |
| Olive Balm Care Ltd | Health & social care | 2025 |
| Olive Balm Properties Ltd | Real estate | 2025 |
| Oliveira-Yang Partners Limited | Real estate | 2024 |
| Oliver Drury Limited | Construction | 2016 |
| Olivia MAC Limited | Manufacturing | 2025 |
| OM2 Care Ltd | Health & social care | 2013 |
| Ombre Investment Partners Limited | Professional & scientific | 2025 |
| OMD Motorcycles Ltd | Wholesale & retail | 2025 |
| Omni-Blu Talis UK Limited | Professional & scientific | 2023 |
| Omniverse Ventures Ltd | Administrative & support | 2024 |
| Omplanning Ltd | Professional & scientific | 2026 |
| ON Hold Communications Limited | Information & communication | 2002 |
| Onaca Building Services Limited | Construction | 2008 |
| One Group (London) Limited | Real estate | 2023 |
| One Neutral Electrical London Limited | Construction | 2021 |
| One Wharf Ltd | Construction | 2011 |
| Onkar Solution Limited | Professional & scientific | 2017 |
| Opara Estates Limited | Real estate | 2016 |
| Open a Nursery - Curtly Ania Ltd | Professional & scientific | 2026 |
| Openfintech Finance and IT Consulting Services Ltd | Information & communication | 2017 |
| Opera Interludes Limited | Arts & recreation | 1991 |
| Orange DOT Limited | Professional & scientific | 2018 |
| Orange Local Ltd | Wholesale & retail | 2021 |
| Orangehouse Audio Visual Limited | Construction | 2011 |
| Orchard Park Construction and Developments Ltd | Construction | 2021 |
| Orchard Park Homes (1LR) Ltd | Real estate | 2025 |
| Orchard Park Homes (BP) Ltd | Hospitality | 2021 |
| Orchard Park Homes (BP2) Ltd | Real estate | 2022 |
| Orchard Park Homes (BRE) Ltd | Real estate | 2021 |
| Orchard Park Homes (CHE) Ltd | Real estate | 2020 |
| Orchard Park Homes (LTE) Limited | Real estate | 2024 |
| Orchard Park Homes (WSE) Ltd | Administrative & support | 2026 |
| Orchard Park Homes Ltd | Real estate | 2011 |
| Orchard Psychology Limited | Professional & scientific | 2015 |
| Orchid Blossom Arts Limited | Professional & scientific | 2025 |
| Origin Chains Ltd | Information & communication | 2025 |
| Original Nerd Committee Ltd | Wholesale & retail | 2023 |
| Ortac International Accounting & Tax Services Limited | Professional & scientific | 2022 |
| Ortaggi Limited | Wholesale & retail | 2007 |
| Orvari Limited | Administrative & support | 2026 |
| Orviller Ltd | Manufacturing | 2014 |
| Osman Investments Ltd | Real estate | 2021 |
| Osman Wealth Management Limited | Professional & scientific | 2015 |
| Ostia Estates Limited | Real estate | 2023 |
| Ostia London Limited | Real estate | 2020 |
| Otto Parade Limited | Hospitality | 2021 |
| Outbound Advisory UK Ltd | Professional & scientific | 2021 |
| Outram Residential Limited | Households | 2022 |
| Outstanding Scaffolding Limited | Construction | 2008 |
| Outwood Court Ltd | Households | 2024 |
| OVE Fulham Ltd | Hospitality | 2026 |