| NINETY1 Consultancy Services Limited | Professional & scientific | 2025 |
| Ninth Quadrant Limited | Information & communication | 2022 |
| Nirva Systems UK Ltd | Information & communication | 2008 |
| Nivin Infotech Limited | Professional & scientific | 2023 |
| NJB Sales Limited | Wholesale & retail | 2012 |
| NJW Investment Ltd | Financial services | 2025 |
| NO Accounting for Taste Ltd | Administrative & support | 2016 |
| NO3 Uppingham Limited | Wholesale & retail | 2015 |
| Noblelift UK Limited | Extraterritorial | 2017 |
| Noka Civil Engineering Ltd | Construction | 2025 |
| Nomis Solutions Ltd | Information & communication | 2024 |
| Noosa Technologies Limited | Professional & scientific | 2018 |
| Norstar Property Maintenance Limited | Households | 2002 |
| North Balmoral Advisory Limited | Professional & scientific | 2024 |
| North Lifting (London) Limited | Construction | 2024 |
| North Star L&D Limited | Professional & scientific | 2019 |
| North Star Project Management Limited | Information & communication | 2008 |
| Northbar (Banbury) Limited | Construction | 2006 |
| Northcourt Industries Ltd | Wholesale & retail | 2020 |
| Northleach House Limited | Real estate | 2020 |
| Northleach House Management Company Limited | Real estate | 2005 |
| Northlight Entertainment Ltd | Information & communication | 2013 |
| Norton Carpentry Limited | Manufacturing | 2022 |
| Nossiter and Barnett Limited | Construction | 2012 |
| Notebook & PEN Limited | Administrative & support | 2022 |
| Nova Brothers Limited | Hospitality | 2023 |
| Nova Elite Construction Limited | Construction | 2024 |
| Novaclima Ltd | Construction | 2025 |
| Novalyte Ltd | Wholesale & retail | 2026 |
| NOW Design Limited | Information & communication | 2008 |
| NOW EV Ltd | Construction | 2021 |
| Nowicki Building Services Limited | Construction | 2018 |
| Nownet Limited | Information & communication | 2014 |
| NPM Resilience Ltd | Information & communication | 2025 |
| NRG Experts Limited | Professional & scientific | 2007 |
| NRS Engineering Ltd | Professional & scientific | 2026 |
| NSB Windows Limited | Construction | 2015 |
| NTD Chauffeur Services Ltd | Transport & logistics | 2008 |
| NTS Property Holdings Ltd | Construction | 2024 |
| NU48 Ltd | Other services | 2026 |
| Number Two Chartmoor Group Limited | Wholesale & retail | 2020 |
| Nurturing Heart Ltd | Health & social care | 2015 |
| NUTRITION1ST Ltd | Health & social care | 2018 |
| Nuvik Global Limited | Administrative & support | 2018 |
| Nuvik UK Ltd | Administrative & support | 2018 |
| Nvest Property Ltd | Real estate | 2023 |
| Nyumbani Ltd | Real estate | 2023 |
| O & C World Limited | Manufacturing | 2024 |
| Oak Bank School | Education | 2013 |
| Oakfield Bookkeeping & Payroll Services Limited | Professional & scientific | 2003 |
| Oakford Technical Services Limited | Professional & scientific | 2018 |
| Oakgrove (Backus Lodge) Management Company Limited | Households | 2013 |
| Oakgrove (Eniac View) Management Company Limited | Households | 2013 |
| Oakgrove (Mixed USE Estate) Residents Management Company Limited | Households | 2013 |
| Oakgrove Phase 5 Management Company Limited | Households | 2018 |
| Oaklands (Reading) Management Association Limited | Households | 1967 |
| Oakman Racing Ltd | Arts & recreation | 2024 |
| Oakridge Park (Leighton Buzzard) Residents Limited | Households | 1999 |
| Oakwood & Sons Ltd | Real estate | 2020 |
| Oakwood Trees & Landscaping Ltd | Administrative & support | 2025 |
| OBE Content & PR Limited | Professional & scientific | 2018 |
| Obtain Business Finance Limited | Financial services | 2024 |
| Octopus Print Solutions Limited | Administrative & support | 2005 |
| Odaat Fitness Ltd | Education | 2021 |
| Office Supplies News Limited | Professional & scientific | 2010 |
| Office to Rent.Com Limited | Professional & scientific | 2004 |
| OJ Media Ltd | Information & communication | 2020 |
| Old Salt Ltd | Professional & scientific | 2018 |
| Old Smoke Vapes Limited | Wholesale & retail | 2018 |
| Oldsport Outfitters Ltd | Agriculture & fishing | 2020 |
| Olfiction Limited | Manufacturing | 2016 |
| Olga Consulting Ltd | Information & communication | 2024 |
| Olivan Homes Ltd | Real estate | 2024 |
| Olive LB Ltd | Hospitality | 2025 |
| Olive Tree Court RTM Company Limited | Households | 2016 |
| Olivia Goodwin Limited | Other services | 2016 |
| Olltom Limited | Professional & scientific | 2008 |
| Olney Building & Renovations Ltd | Construction | 2025 |
| OLU ADE Care Home Limited | Health & social care | 2025 |
| OM Car Parts Ltd | Wholesale & retail | 2022 |
| Om's Burgers Ltd | Hospitality | 2024 |
| Omatra Limited | Information & communication | 2020 |
| Omedex Ltd | Manufacturing | 2004 |
| Omiibea Limited | Wholesale & retail | 2022 |
| Omnigss Limited | Financial services | 2018 |
| ON & OFF Electrical and Building Ltd | Construction | 2015 |
| ON Cloud 9 Events Limited | Hospitality | 2025 |
| ON the Level (WET Rooms) Limited | Manufacturing | 2007 |
| On-Setlab Limited | Information & communication | 2010 |
| One Big Triangle Ltd. | Professional & scientific | 2018 |
| One Cinema Network Ltd | Information & communication | 2026 |
| One Degree NOW Ltd | Professional & scientific | 2016 |
| One Stop MOT Shop Ltd | Wholesale & retail | 2012 |
| One Stretch Beyond Limited | Information & communication | 2013 |
| One-Iron Limited | Agriculture & fishing | 1985 |
| Onea Appliances Limited | Other services | 2025 |
| Onebyte Studio Limited | Information & communication | 2025 |
| Onesixteen Limited | Real estate | 2018 |
| Onestm Ltd | Construction | 2023 |
| Onesun Technologies Limited | Hospitality | 2006 |