| North London Structures and Groundworks Ltd | Construction | 2019 |
| North Star Practice Ltd | Professional & scientific | 2023 |
| North Street Development Co Ltd | Real estate | 2021 |
| North Street News & Booze Limited | Wholesale & retail | 2018 |
| North Weald Aviation Support Services Limited | Transport & logistics | 2022 |
| Northgate Primary School Trust | Education | 2012 |
| Northhjune Ltd | Information & communication | 2025 |
| Northreach Limited | Administrative & support | 2015 |
| Northreach Property Services Limited | Construction | 2016 |
| Northstar Managed Services Ltd | Information & communication | 2012 |
| NOT Robots Consulting Limited | Professional & scientific | 2024 |
| Notionlink Limited | Professional & scientific | 1990 |
| Notlem (Herts) Limited | Construction | 2013 |
| Notley Management Ltd | Professional & scientific | 2018 |
| Nottage Productions Ltd | Information & communication | 2013 |
| Nouve Limited | Wholesale & retail | 2026 |
| Nova Ascend Collective Limited | Professional & scientific | 2025 |
| Novartec Ltd | Professional & scientific | 2022 |
| Novosapien Limited | Information & communication | 2025 |
| Novus Compliance Services Limited | Administrative & support | 2012 |
| NP Creative Design Ltd | Professional & scientific | 2021 |
| NP Essex Construction Limited | Construction | 2011 |
| NPS Electrical & SON Ltd | Construction | 2021 |
| Nquix Ltd | Professional & scientific | 2011 |
| NRJ P Capital Ltd | Financial services | 2022 |
| NT Whitaker Limited | Real estate | 2018 |
| Nulty Cader Limited | Professional & scientific | 2015 |
| Nursery Grove Limited | Households | 2020 |
| Nurturing Pregnancy Limited | Other services | 2020 |
| Nyjproperty Ltd | Real estate | 2023 |
| Nysa Digital Limited | Information & communication | 2023 |
| Nytes Ltd | Manufacturing | 2020 |
| O & E Ventures Limited | Professional & scientific | 2015 |
| O M Deehan & SON Ltd | Water & waste | 2019 |
| O Triple Limited | Information & communication | 2024 |
| O&G Cleaning Ltd | Administrative & support | 2019 |
| O'Neill and Associates Ltd | Administrative & support | 2017 |
| OA 5.COM Limited | Information & communication | 2000 |
| Oak Hall Property Limited | Households | 1996 |
| Oak Recruitment Partnership Limited | Administrative & support | 2023 |
| Oakbridge Workforce Solutions Ltd | Administrative & support | 2024 |
| Oakfield Bathrooms Limited | Professional & scientific | 2008 |
| Oakhaven Tax & Accounting Ltd | Professional & scientific | 2025 |
| Oakridge Capital Partners Ltd | Other services | 2025 |
| Obsidian Financial Services Limited | Financial services | 2025 |
| Obsidian INK Ltd | Arts & recreation | 2022 |
| Oertekyr Limited | Wholesale & retail | 2026 |
| OFF the Hook Marketing Ltd | Professional & scientific | 2024 |
| Office FOX Limited | Administrative & support | 2016 |
| Ofmsignatures Suites Ltd | Real estate | 2023 |
| Ohba Leaf BS Limited | Hospitality | 2023 |
| Ohmii Ltd | Other services | 2025 |
| Oisoy Ltd | Extraterritorial | 2022 |
| OJA Nutrition Ltd | Health & social care | 2024 |
| OK Records Ltd | Information & communication | 2020 |
| Old Park Meadow Limited | Other services | 2016 |
| Olivaris Limited | Arts & recreation | 1997 |
| Olive & Oak Carpentry Ltd | Manufacturing | 2025 |
| Oliver Last Limited | Other services | 2019 |
| Olivia Van DER Horst Ltd | Other services | 2019 |
| Oljic Limited | Professional & scientific | 2011 |
| Olsi Investment Limited | Other services | 2025 |
| Olumia Limited | Health & social care | 2016 |
| Olyra Greek Bakehouse Ltd | Manufacturing | 2020 |
| Omar Foster Ltd | Hospitality | 2025 |
| Ombler Consultancy Limited | Professional & scientific | 2024 |
| Omega Process Equipment Limited | Wholesale & retail | 1981 |
| Omega Property Investors Ltd | Real estate | 2019 |
| Omega Valves Limited | Extraterritorial | 1986 |
| Onchain Frontyr Limited | Information & communication | 2026 |
| Oncoart Limited | Health & social care | 2015 |
| One Components Ltd | Wholesale & retail | 2022 |
| One Fabrics Limited | Other services | 2023 |
| One Hundred Proof Ltd | Information & communication | 2009 |
| One-Pack Solutions Ltd | Professional & scientific | 2005 |
| Onexius Ltd | Professional & scientific | 2019 |
| Onset Brand Consultancy Limited | Professional & scientific | 2018 |
| Ontrack International Ltd | Professional & scientific | 1990 |
| Onyx Holidays Limited | Administrative & support | 2025 |
| Open Edge Group Ltd | Construction | 2024 |
| Opera Jewellers in London Limited | Manufacturing | 2010 |
| Opstead Limited | Professional & scientific | 2018 |
| Optim Trans Ltd | Transport & logistics | 2019 |
| Optimal Training & Consultancy Ltd | Extraterritorial | 2019 |
| Optimus Learning Services Limited | Administrative & support | 1991 |
| Oram Court (Spellbrook) Management Company Limited | Households | 2013 |
| Orama Management Limited | Information & communication | 2009 |
| Orange Lighting Limited | Professional & scientific | 2008 |
| Orazon Ltd | Information & communication | 2025 |
| Orbis Archive Research Ltd | Administrative & support | 2016 |
| Orchard Walk Residents Management Company Limited | Households | 1999 |
| Orchid Healthcare Ltd | Administrative & support | 2015 |
| Ordo Consulting Ltd | Professional & scientific | 2013 |
| Organic Magic Ltd | Extraterritorial | 2024 |
| Oricor for MEN Ltd | Wholesale & retail | 2026 |
| Originate Ltd | Information & communication | 2021 |
| Origo Investment Limited | Administrative & support | 2000 |
| OS Electrical Contractors Ltd | Construction | 2026 |
| OSB & Sons Ltd | Transport & logistics | 2023 |
| Oscar Charlie Comms Limited | Professional & scientific | 2016 |