| North Herts Taxis Ltd | Transport & logistics | 2020 |
| North Road Ltd | Professional & scientific | 2018 |
| Northcott Electronics Limited | Wholesale & retail | 1987 |
| Northolme Limited | Professional & scientific | 2004 |
| Northwood Initiatives Ltd | Professional & scientific | 2016 |
| Norton Michael Financial Management Limited | Financial services | 2014 |
| Norton Road Sport and Leisure Limited | Arts & recreation | 1995 |
| Nostrada AI Ltd | Information & communication | 2025 |
| Notional Limited | Professional & scientific | 2007 |
| Nott Pybus & Associates Limited | Financial services | 2000 |
| Nova Project Group Ltd | Other services | 2021 |
| Nova Technology Systems Ltd | Information & communication | 2025 |
| Novascape Solution Limited | Professional & scientific | 2025 |
| Novavivo Limited | Real estate | 2018 |
| November North Ltd | Education | 2023 |
| Noventura Limited | Professional & scientific | 2025 |
| Nrgee Ltd | Wholesale & retail | 2009 |
| Ntsama Chilli OIL and Sauces Limited | Manufacturing | 2017 |
| Nugent Wiles Limited | Real estate | 1996 |
| Number 12 Coffee Letchworth Limited | Hospitality | 2025 |
| Number 56 Limited | Other services | 2021 |
| Number Crunching Limited | Professional & scientific | 2006 |
| Numerical Fluency Holdings Limited | Financial services | 2026 |
| Numerical Fluency Limited | Education | 2020 |
| Numismatics Solutions Ltd | Financial services | 2021 |
| Nunn IP Services Ltd. | Professional & scientific | 2026 |
| Nurturecare Support Limited | Health & social care | 2021 |
| Nutley Investments Limited | Real estate | 2025 |
| Nutley's Barbers Limited | Other services | 2024 |
| Nutley's Tanning Limited | Other services | 2024 |
| Nwccs Limited | Financial services | 2021 |
| NX JEN Ltd | Administrative & support | 2021 |
| Nyenyezi Ltd | Arts & recreation | 2022 |
| NZE Limited | Public administration | 2023 |
| O Fursland Carpentry Limited | Manufacturing | 2017 |
| O'Brien Construction and Development Limited | Construction | 2014 |
| O'Dell Rework Solutions Limited | Manufacturing | 2009 |
| O'Hara & Sons Ltd | Hospitality | 2024 |
| O'Sarracino Cafe Limited | Hospitality | 2024 |
| Oak Lodge Enterprises Limited | Professional & scientific | 2021 |
| Oak Tree Car Sales Ltd | Wholesale & retail | 2018 |
| Oak Tree Property and Real Estate Ltd | Real estate | 2021 |
| Oakfield Projects Ltd | Professional & scientific | 2025 |
| Oakfield Service Company Limited | Administrative & support | 2016 |
| Oakwood Assets Limited | Real estate | 2015 |
| OB Property Solutions Ltd | Administrative & support | 2025 |
| Oblong PUB Company Ltd | Hospitality | 2017 |
| Ocaro Ltd | Financial services | 2021 |
| Occupational Psychology Matters Limited | Professional & scientific | 2007 |
| Ocean Mammal Ltd | Wholesale & retail | 2024 |
| Ocean Objective Ltd | Professional & scientific | 2024 |
| Oden Partnership Limited | Information & communication | 2004 |
| Ofayo Limited | Education | 2010 |
| Ofdl Biomass Limited | Energy | 2015 |
| Ofdl Holdings Limited | Financial services | 2017 |
| OFF the Wool Limited | Manufacturing | 2014 |
| OFF World Films Limited | Information & communication | 2017 |
| Office Assure Limited | Information & communication | 2016 |
| Offley Grange Equine Limited | Agriculture & fishing | 2017 |
| Offley Grange Limited | Real estate | 2025 |
| Offley HOO Bridal Ltd | Wholesale & retail | 2026 |
| Offscript Content Ltd | Professional & scientific | 2021 |
| Ogun Advisory Ltd | Professional & scientific | 2025 |
| Ogun Sovereign Wealth Ltd | Financial services | 2025 |
| OH Medical Holdings Ltd | Financial services | 2024 |
| OH Medical Recruitment Ltd | Administrative & support | 2018 |
| Ohto Experiential Limited | Professional & scientific | 2023 |
| OI Pharma Partners Ltd | Professional & scientific | 2010 |
| OJC Technologies Limited | Information & communication | 2011 |
| OJW Media Ltd | Information & communication | 2026 |
| Okavango Property Investments Ltd | Real estate | 2020 |
| OKE (Catering Equipment & Ventilation Systems) Ltd | Hospitality | 2004 |
| Oktar Limited | Professional & scientific | 2024 |
| OLA & OLA Ltd | Health & social care | 2022 |
| Old Hall Farmyard RES Ltd | Construction | 2015 |
| Old School (Lilley) Management Company Limited | Households | 1989 |
| Old Stables (Roxley Court) Management Co Ltd | Households | 2003 |
| Oldchurch Consultants Limited | Financial services | 2010 |
| Olexon Ltd | Mining & quarrying | 2025 |
| OLI Metcalfe Design Ltd | Professional & scientific | 2012 |
| Oliva Construction Management Ltd | Construction | 2013 |
| Oliver & Oliver Consultants Ltd | Other services | 2023 |
| Oliver Door Company Limited | Construction | 2003 |
| Oliver Russell Electrical Limited | Construction | 2012 |
| Oliver Wright Limited | Administrative & support | 2000 |
| Olmgmt Ltd | Wholesale & retail | 2025 |
| Ology Academy Ltd | Education | 2018 |
| Ology Medspa Limited | Other services | 2019 |
| Omanos Analytics Ltd | Information & communication | 2017 |
| Omega Environmental Services Limited | Water & waste | 2012 |
| Omnibase Ltd | Information & communication | 2025 |
| ON Top Ltd | Financial services | 2014 |
| Once There WAS a Girl Ltd | Professional & scientific | 2022 |
| Oncloud Europe Ltd | Professional & scientific | 2023 |
| One Good Thing Group Ltd | Financial services | 2023 |
| One Good Thing Ltd | Manufacturing | 2022 |
| One Simple Change Consulting Limited | Professional & scientific | 2024 |
| Onex Autos Ltd | Wholesale & retail | 2023 |
| Onito Ltd | Information & communication | 2026 |
| Onlineshopbase Limited | Transport & logistics | 2013 |