| North Star Preschool Ltd | Health & social care | 2018 |
| Northbound Advisory Ltd | Professional & scientific | 2026 |
| Northern Crystaltech UK Ltd | Wholesale & retail | 2023 |
| Northreach Properties Limited | Real estate | 2008 |
| Northstar Talent Ltd | Administrative & support | 2026 |
| Norvec Limited | Information & communication | 2026 |
| NOT From Ltd | Wholesale & retail | 2016 |
| Nott Property Ltd | Real estate | 2026 |
| Notting Hill Gate Filling Station Ltd | Wholesale & retail | 2021 |
| Nova Global International GB Ltd | Transport & logistics | 2022 |
| Novabuild Construction Ltd | Construction | 2025 |
| Novigo Limited | Administrative & support | 2012 |
| Novo Energy Limited | Professional & scientific | 2013 |
| Novo Energy Services Ltd | Professional & scientific | 2022 |
| NP Buildings & Engineering Limited | Construction | 2014 |
| NP Holdings & Associates Limited | Real estate | 2022 |
| NS Roofing Limited | Construction | 2024 |
| NSF Developments Ltd | Construction | 2026 |
| NT Innovations Ltd | Information & communication | 2023 |
| NTC53 Limited | Wholesale & retail | 2018 |
| Nu-Life Holdings Limited | Education | 2016 |
| Nu-Life Surgery Ltd | Health & social care | 2012 |
| Nuance Creative Ltd | Wholesale & retail | 2021 |
| Nuclius Ltd | Information & communication | 2025 |
| Nugawi Limited | Professional & scientific | 2023 |
| Nukak U.K. Ltd | Wholesale & retail | 2021 |
| Number 9 Cornwall RD Freeholders Limited | Households | 2020 |
| Numbers for Impact Limited | Professional & scientific | 2023 |
| Nupur Investments Ltd | Real estate | 2021 |
| Nutruit Ltd | Wholesale & retail | 2022 |
| NWC Distribution Ltd | Wholesale & retail | 2025 |
| Nxcodes Ltd | Information & communication | 2018 |
| O & O Associates Ltd | Real estate | 2018 |
| O H Practice Limited | Health & social care | 2008 |
| O Scarfe Flooring Limited | Construction | 2014 |
| O'Connor Advisory Ltd | Professional & scientific | 2011 |
| Oak IT Consulting Limited | Information & communication | 2021 |
| Oak Support Limited | Health & social care | 2021 |
| Oakdale Flat Management Limited | Households | 1996 |
| Oakfield Career Services Limited | Information & communication | 2025 |
| Oakhanger Properties Limited | Real estate | 1996 |
| Oakmoor Heating Limited | Construction | 2015 |
| Oakshall Limited | Wholesale & retail | 2023 |
| Oasis Watering Ltd. | Professional & scientific | 2010 |
| OB Coaching Space Ltd | Education | 2026 |
| Obelisk Court Ltd | Households | 2006 |
| Oberhummer Consulting Ltd | Wholesale & retail | 2026 |
| OBLIG8 Limited | Professional & scientific | 2024 |
| Obrien & Sons Roofing Ltd | Construction | 2025 |
| Obsgynae Healthcare UK Ltd | Health & social care | 2014 |
| OBT2 Investment Limited | Real estate | 2022 |
| Oceanic Taste Limited | Hospitality | 2023 |
| Oceanus Leadership Development Ltd | Professional & scientific | 2024 |
| Ocfo Consulting Limited | Professional & scientific | 2023 |
| Octain Construction Limited | Construction | 1994 |
| Octain Developments Limited | Construction | 2000 |
| Octain Lettings Ltd | Real estate | 2022 |
| OFF the MAP Productions Limited | Information & communication | 2010 |
| OFF the Record (Twickenham) Ltd. | Health & social care | 2013 |
| Off-Piste the Movie Limited | Information & communication | 2014 |
| Offshoot Holdings Ltd | Manufacturing | 2020 |
| Ofori Hammond Ltd | Health & social care | 2011 |
| Ogden Consulting Limited | Professional & scientific | 2017 |
| OIL and GAS London Ltd | Other services | 2014 |
| Old Courts (Management) Limited (the) | Households | 1966 |
| Old Manor Consulting Ltd | Information & communication | 2015 |
| Old Maps Limited | Arts & recreation | 2015 |
| Olive Asset Management Ltd | Extraterritorial | 2024 |
| Oliver Malcolm Publishing Transformation Ltd | Information & communication | 2024 |
| Oliver Trading Limited | Hospitality | 2018 |
| Olma Ltd | Extraterritorial | 2023 |
| OLP Holdco Limited | Financial services | 2021 |
| OLP Limited | Real estate | 2006 |
| OM Kapital Ltd | Financial services | 2025 |
| Omnia Learning Trust | Education | 2013 |
| ON Q Solutions Limited | Professional & scientific | 2011 |
| ON the Line Film Limited | Information & communication | 2022 |
| ON Time Pest Control Limited | Administrative & support | 2018 |
| One Afternoon Ltd | Wholesale & retail | 2023 |
| One Centillion Ltd | Real estate | 2023 |
| One Health Clinic Ltd | Health & social care | 2022 |
| One Lebanon Park Mansions Ltd | Households | 2006 |
| One London Property Ltd | Professional & scientific | 2015 |
| One Moorgate Limited | Professional & scientific | 2019 |
| One Network Express Ltd | Transport & logistics | 2025 |
| One Stitch Beyond Limited | Wholesale & retail | 2012 |
| Onesec Management Ltd | Professional & scientific | 2013 |
| Onestage Limited | Administrative & support | 2000 |
| Onestep Earth Limited | Wholesale & retail | 2018 |
| Online Earth Ltd | Information & communication | 2020 |
| Online Garden Design Ltd | Professional & scientific | 2023 |
| Onlyshine Limited | Administrative & support | 2025 |
| Onroof Ltd | Construction | 2026 |
| Onward Engineering Consultants Limited | Professional & scientific | 2025 |
| Onward Engineering Limited | Professional & scientific | 2012 |
| Openfare Ltd | Transport & logistics | 2026 |
| OPTIONS2BUY Limited | Education | 2003 |
| Optivate Ltd | Other services | 2013 |
| Opus Sun Ltd | Information & communication | 2010 |
| Opyn Consulting Limited | Professional & scientific | 2023 |