| NSL Limited | London | 2006 |
| Nsofors Limited | Ellesmere Port | 2024 |
| NSS Security Systems Limited | Harold Hill | 2011 |
| Nssl Group Ltd | Borehamwood | 2023 |
| NTR Admin & Consulting Ltd | Ipswich | 2023 |
| NTT Safety Services Limited | Shepperton | 2013 |
| NU Health MED Limited | Hull | 2026 |
| NU YOU Medical Limited | Billericay | 2024 |
| Nuclear Liabilities Fund Limited | Edinburgh | 1996 |
| Number 01 Limited | London | 2019 |
| Numenow Ltd | Romford | 2021 |
| NUR Eudemonia Ltd | London | 2020 |
| Nuralife Ltd | Camberley | 2026 |
| Nursing Academy Limited | London | 2022 |
| Nurtura Healthcare Ltd | Falkirk | 2025 |
| Nurture Care Inspections Limited | Lancaster | 2025 |
| Nurture Health and Care Ltd | Norwich | 2021 |
| Nurture Healthcare Solutions Ltd | Sunderland | 2023 |
| Nurture Nest Care Ltd | Slough | 2024 |
| Nurtureganics Limited | Houghton Regis | 2025 |
| Nurturing Somerset Together Limited | Chard | 2025 |
| Nutrileague Limited | Ilford | 2022 |
| Nutrition Advisory Team Limited | Poole | 2020 |
| Nutrition and YOU Ltd | Hayes | 2020 |
| Nutrition in CIC | Keighley | 2024 |
| Nutritional Health Enterprise Ltd | London | 2022 |
| Nvizion Ltd | London | 2019 |
| NW Fire Ltd | Newtownabbey | 2012 |
| NW Firecontrol Limited | Warrington | 2007 |
| NW Planning & Operations Ltd | Erith | 2025 |
| Nwaf Limited | London | 2026 |
| Nwfss Ltd | Londonderry | 2024 |
| NX Medical Ltd | Dudley | 2022 |
| Nyadowa Services Limited | South Mimms | 2019 |
| Nyame Care Ltd | London | 2025 |
| NYC Limited | Ilford | 2023 |
| NYG Safety Ltd | Coventry | 2020 |
| Nyoung Limited | Sidcup | 2023 |
| NYS Home Care Ltd | Preston | 2022 |
| NYS Security Ltd | London | 2025 |
| Nytagystix Ltd | London | 2023 |
| NZE Limited | Hitchin | 2023 |
| Nzwicl Ltd | Bristol | 2023 |
| O&A Contracts Ltd | Tring | 2020 |
| O'Brien Fire Consultancy Ltd | Bury | 2022 |
| O'BRIEN88 Business Services Limited | Warrington | 2018 |
| Oak Care Home Limited | Enfield | 2021 |
| Oak Pharma Ltd | Walsall | 2022 |
| Oak View Family Assessment Centre Limited | Reading | 2020 |
| Oakcrest Care Ltd | Bolton | 2025 |
| Oakes Design Limited | Windsor | 2011 |
| Oaklea Carers Ltd | Farnborough | 2025 |
| Oakmont Care Ltd | Peterborough | 2018 |
| Oakpark Alarms Security Services Limited | Buckingham | 2002 |
| Oakwell Care Ltd | Southsea | 2026 |
| Oasis Care-Uk Limited | Chelmsford | 2016 |
| Oasis Plus Care Services Ltd | Morden | 2020 |
| OBERT20 Ltd | Southampton | 2024 |
| Obhealt Ltd | Cardiff | 2022 |
| Obi-Alpha Limited | London | 2024 |
| OBM Onelab CIC | Ilford | 2018 |
| Obsidian Fire Engineering Solutions Ltd | Bradford | 2025 |
| OC Ortho Ltd | Fulford | 2026 |
| Ocala Healthcare Limited | Stowmarket | 2017 |
| OCB Site Services Ltd | London | 2005 |
| Occupational Safety and Health Consulting Limited | Morpeth | 2013 |
| Occupational Therapy and Care Services Limited | Berkshire | 2021 |
| Occupational Therapy Pathways to Potential Ltd | Teignmouth | 2025 |
| Ocean Care Residence Marvel House Ltd | Peterborough | 2026 |
| Oceanic Security Services Ltd | London | 2021 |
| Oceanside Services Limited | East Tilbury | 2020 |
| Ocelot Fire Consultancy Limited | Bradford | 2022 |
| Octane Avios Group Ltd | Coventry | 2023 |
| October 16 Consulting Limited | Goole | 2020 |
| Octobyte Limited | Stevenage | 2019 |
| ODB Engineering Ltd | Strabane | 2024 |
| ODC (Defence) Limited | Coventry | 2014 |
| Odessa International Brokerage Limited | Taunton | 2025 |
| Odgconsulting Limited | Southampton | 2022 |
| Odiase Services Limited | Preston | 2018 |
| Odiliaclark Ltd | Letchworth Garden City | 2018 |
| Odin Defence Ltd | London | 2024 |
| ODO Ayaresa Care Ltd | Milton Keynes | 2023 |
| ODP Express Ltd | Barnsley | 2023 |
| Odyssey Fire Safety Ltd | London | 2023 |
| OEK Ltd. | London | 2018 |
| OEL Paraplanning Solutions Ltd | London | 2023 |
| Ofac Limited | Oxford | 2023 |
| Ofbad Resources Limited | Ashton-Under-Lyne | 2018 |
| OFF Stage Strategies Limited | Maidstone | 2024 |
| Offender Manager Expertise Ltd | Stourport-On-Severn | 2020 |
| Office Limited | Fareham | 2025 |
| Offsolutions Ltd | Clydebank | 2024 |
| OFR Holdings Limited | Manchester | 2021 |
| Ogomide Limited | Caerdydd | 2023 |
| OH2U Ltd | Liverpool | 2024 |
| Ohch Ltd | Birmingham | 2025 |
| OHR Services Ltd | Peterborough | 2020 |
| OIL and GAS Authority | London | 2015 |
| OJ Carter Limited | Birmingham | 2025 |