| Northern Lights Occasions Ltd | Wholesale & retail | 2025 |
| Northfour Limited | Wholesale & retail | 2026 |
| Northolt Rugby Limited | Arts & recreation | 2010 |
| Norton Five Holdings Limited | Financial services | 2025 |
| Norton Five Limited | Information & communication | 2017 |
| Norwood Properties Limited | Real estate | 1993 |
| Norwoods Accountancy Limited | Professional & scientific | 2016 |
| NOT Another Diet Club Ltd | Other services | 2024 |
| Notsoshy Ltd | Wholesale & retail | 2022 |
| Nour Atelier Limited | Wholesale & retail | 2025 |
| Nova Balloons Ltd | Hospitality | 2025 |
| Novacloak Limited | Wholesale & retail | 2023 |
| Novacroft IT Limited | Professional & scientific | 2006 |
| Novar (Stelrad) Limited | Other services | 1988 |
| Novar (Twyfords) Limited | Professional & scientific | 1896 |
| Novar Ed&S Limited | Manufacturing | 1923 |
| Novar Electrical Holdings Limited | Professional & scientific | 1965 |
| Novar Europe Limited | Financial services | 1955 |
| Novar Holdings Limited | Professional & scientific | 1985 |
| Novar Limited | Professional & scientific | 1988 |
| Novar Systems Limited | Administrative & support | 1932 |
| Novel Bound Events Ltd | Administrative & support | 2024 |
| Novemnine Ltd | Professional & scientific | 2015 |
| Novtile Ltd | Construction | 2017 |
| NRG Core Global (UK) Limited | Administrative & support | 2024 |
| NRG Heating Specialist Ltd | Construction | 2016 |
| Ntech Engineering Consultants Limited | Professional & scientific | 2022 |
| NTK Formwork Ltd | Construction | 2020 |
| Nu-Gen HR Ltd | Professional & scientific | 2022 |
| Nugents Waste Management Services Limited | Water & waste | 2015 |
| Number Cruncher Business Services Limited | Administrative & support | 2008 |
| Numeracy Ltd | Financial services | 2009 |
| Nuralis Ltd | Information & communication | 2025 |
| Nurture Public Relations Limited | Professional & scientific | 2026 |
| Nyamefam Limited | Administrative & support | 2025 |
| Nyble Ltd | Information & communication | 2020 |
| O'Brien Marine Limited | Extraterritorial | 2022 |
| O2NOBLETECH Limited | Information & communication | 2024 |
| Oak and Stone Landscapes Ltd | Administrative & support | 2022 |
| Oak Tree Cricket Operations Ltd | Arts & recreation | 2024 |
| Oak Tree Farm Holdings (Binfield) Ltd | Construction | 2024 |
| Oakberry Holdings Limited | Financial services | 2025 |
| Oakley Clinical Solutions Ltd | Professional & scientific | 2025 |
| Oakley Jane Property Services Limited | Real estate | 2022 |
| Oaktree Garden Centre Online Limited | Wholesale & retail | 2013 |
| Oaktree Garden Nursery LLP | | 2024 |
| Oaktree Nursery Ltd | Agriculture & fishing | 2021 |
| Oakwood International Ltd | Professional & scientific | 2002 |
| OAT PUB Company Ltd | Hospitality | 2020 |
| Oberoi Dispenza Consulting Limited | Information & communication | 2014 |
| Oberoi Dispenza Property Limited | Real estate | 2019 |
| Objective Professional Services Limited | Administrative & support | 1994 |
| Ocean Planning and Consulting Ltd | Construction | 2021 |
| Ocean Spray International Services (U.K.) Ltd. | Administrative & support | 1995 |
| Oceanport Trading International Limited | Mining & quarrying | 2013 |
| Oceanport Trading Limited | Administrative & support | 2001 |
| Octain Applied Processing Ltd | Information & communication | 2022 |
| Octave AI Limited | Information & communication | 2023 |
| Odessos Limited | Manufacturing | 2006 |
| Odyssey Education Services Limited | Education | 2018 |
| Oeuvre a Participatory Service Ltd | Arts & recreation | 2021 |
| Off-Grid Electrical Limited | Construction | 2022 |
| OGZ Supplies Ltd | Wholesale & retail | 2023 |
| OH for FUR Coats Ltd | Other services | 2024 |
| OH SO Keen Ltd | Professional & scientific | 2026 |
| Ohana Education Services Limited | Education | 2025 |
| Ohsher Ltd | Professional & scientific | 2018 |
| OI ZOI OI Limited | Wholesale & retail | 2013 |
| OIL and GAS Epcm Ltd | Professional & scientific | 2017 |
| Okanargon Limited | Real estate | 1974 |
| Okto Building Limited | Construction | 2019 |
| Ola-Ojo Limited | Health & social care | 2021 |
| Olay Consult Ltd | Professional & scientific | 2015 |
| Oles Property Ltd | Real estate | 2024 |
| Olive UK Opco Limited | Financial services | 2020 |
| Oliverfinn Trade Limited | Wholesale & retail | 2023 |
| OMBZ2020 Limited | Professional & scientific | 2020 |
| Omega Build Ltd | Construction | 2024 |
| Omega Security Limited | Administrative & support | 2002 |
| Omega Utility Services Limited | Information & communication | 2021 |
| Omex Technology Systems 2022 Ltd | Administrative & support | 2022 |
| ON Point Property Investments Ltd | Construction | 2019 |
| ON Wing Graphics Ltd | Professional & scientific | 2025 |
| Once Upon a BUS Limited | Education | 2013 |
| One Application Ltd | Information & communication | 2016 |
| One Call Health Ltd | Health & social care | 2023 |
| One Contractors (Specialist Electrical Contractors) Limited | Construction | 2018 |
| One Degree North Ltd | Real estate | 2019 |
| One Future Education Ltd | Education | 2021 |
| One Zero Limited | Information & communication | 1996 |
| Onecall City Ltd | Administrative & support | 2025 |
| Onekallworks Ltd | Administrative & support | 2026 |
| Onemedia Holdings Limited | Financial services | 2023 |
| Onemedia Services Limited | Wholesale & retail | 2008 |
| Onity Limited | Professional & scientific | 2000 |
| Online Treasures Ltd | Real estate | 2020 |
| Onto Bespoke Packaging Limited | Professional & scientific | 2018 |
| Open Innovation AI Ltd | Information & communication | 2023 |
| Open Landscapes and Family Ltd | Administrative & support | 2024 |
| Opencfd Limited | Information & communication | 2004 |