| Northgate (CB 2) Limited | Other services | 2012 |
| Northgate (CB) Limited | Professional & scientific | 2010 |
| Northgate (Europe) Limited | Financial services | 2006 |
| Northgate Fleet Services Limited | Administrative & support | 1992 |
| Northgate Highways Limited | Manufacturing | 1995 |
| Northgate Mobility Limited | Information & communication | 1996 |
| Northgate Temperature Controlled Limited | Wholesale & retail | 2008 |
| Northgate Vehicle Hire Limited | Administrative & support | 1979 |
| Northgate Vehicle Sales Limited | Wholesale & retail | 1989 |
| Northside Legacy Limited | Wholesale & retail | 2025 |
| Northstar Heating Ltd | Construction | 2023 |
| Northumberland Luxury Accommodation Limited | Hospitality | 2023 |
| Nosh and Nectar Ltd | Hospitality | 2024 |
| Nouveau Medica Ltd | Health & social care | 2021 |
| Nouvo Properties Ltd | Households | 2025 |
| Nova Construction & Site Services Ltd | Construction | 2025 |
| Novemcore Limited | Information & communication | 2026 |
| Novo Mortgages Ltd | Financial services | 2024 |
| NOW and Then Darlington Ltd | Wholesale & retail | 2019 |
| NPB Capital Limited | Education | 2024 |
| NPB Group Limited | Financial services | 2024 |
| NPW Engineering Ltd | Construction | 2006 |
| Nrjglobal Ltd | Manufacturing | 2024 |
| NRRP8 Ltd | Hospitality | 2025 |
| Ns-Teck Limited | Water & waste | 2025 |
| NSA (Ventures) Limited | Administrative & support | 2001 |
| NSG Limited | Administrative & support | 1985 |
| NSG Property Services Limited | Households | 1999 |
| Nsody Limited | Wholesale & retail | 2026 |
| NT Enterprises Limited | Financial services | 1997 |
| Ntcdb Retail Limited | Wholesale & retail | 2004 |
| Nu-Interiors Ltd | Construction | 2013 |
| Number One Champagne Bar Limited | Hospitality | 2016 |
| Nunnery Lane (Darlington) Ltd | Hospitality | 2009 |
| Nunnery Plumbing and Heating Ltd | Construction | 2021 |
| Nutri-Prep Foods Ltd | Hospitality | 2018 |
| Nutshell Services Limited | Professional & scientific | 2018 |
| NW Fabrication Limited | Manufacturing | 2025 |
| NWD Accountancy Limited | Professional & scientific | 2015 |
| Nxtgen Sports Coaching Limited | Education | 2026 |
| Nyka Consultancy Limited | Health & social care | 2021 |
| O Mmxxi Limited | Hospitality | 2021 |
| Oak Tree Shared Services Limited | Households | 2014 |
| Oakbridge Property NE Ltd | Real estate | 2022 |
| Oakbridge Property Solutions Ltd | Real estate | 2020 |
| Oakdale Building and Construction Ltd | Construction | 2026 |
| Oakdale Landscapes Ltd | Administrative & support | 2026 |
| Oakdene Management Consultants Ltd | Professional & scientific | 2021 |
| Oakfit Ltd | Manufacturing | 2022 |
| Oakland (Barnard Castle) Management Company Limited | Households | 2004 |
| Oaklea Court (Darlington) Management Company Limited | Households | 1989 |
| Oakmont Dean Ltd | Real estate | 2017 |
| Oaktree Asset Management UK Ltd | Professional & scientific | 2021 |
| Oaktree Home Improvements Ltd | Wholesale & retail | 2019 |
| Oakvale Business Services Limited | Professional & scientific | 2006 |
| Oakwood Construction (Darlington) Limited | Construction | 2008 |
| Oakwood House Property and Developments (UK) Limited | Real estate | 2006 |
| Oakworth Consultancy (Hull) Limited | Financial services | 2007 |
| Oasis Vale Limited | Wholesale & retail | 2025 |
| OC Restaurant Limited | Hospitality | 2017 |
| Occam's Scalpel Ltd | Information & communication | 2022 |
| Ocean Academy Limited | Arts & recreation | 2025 |
| Ocean Fibre Ltd | Information & communication | 2020 |
| Ocean-Tec Systems Limited | Administrative & support | 2006 |
| Oceanex Ltd | Construction | 2011 |
| Oceanic Airlines Limited | Transport & logistics | 2022 |
| Octopus Cleaning Group Ltd | Administrative & support | 2020 |
| Octopus Printing Limited | Professional & scientific | 2017 |
| Octopus Virtual Assistant Ltd | Administrative & support | 2018 |
| ODP Limited | Health & social care | 2015 |
| OEL Technical Services Limited | Construction | 2021 |
| OFF Season Barbers Ltd | Other services | 2019 |
| Office Assist Ltd | Administrative & support | 2022 |
| Offline Network Services Limited | Information & communication | 2007 |
| Offshore Access Solutions Ltd | Construction | 2026 |
| Offshore Accountants Limited | Professional & scientific | 2014 |
| Offshore Engineering Consultants Ltd | Financial services | 2013 |
| OH Specialists Hub Ltd | Health & social care | 2026 |
| OHG Contracting NW Limited | Construction | 2023 |
| Ohmic Training Solutions Limited | Education | 2015 |
| OIL Ridge Ltd | Professional & scientific | 2013 |
| Oink Properties Ltd | Real estate | 2025 |
| Okevery Ltd | Wholesale & retail | 2025 |
| Olavia Ltd | Transport & logistics | 2020 |
| Old Eltringham Court Management Company Limited | Households | 2006 |
| Old School Boxing Darlington Limited | Arts & recreation | 2024 |
| Old School Staindrop Management Limited | Households | 2018 |
| Oliphant Surgical Ltd | Health & social care | 2019 |
| Olivia Coatings Ltd | Construction | 2025 |
| Olivia Salkeld Limited | Wholesale & retail | 2025 |
| Olusoftware Ltd | Information & communication | 2018 |
| Olympia Group (NE) Ltd | Real estate | 2025 |
| Olympia Property Solutions Limited | Construction | 2024 |
| Omni-Shop Ltd | Wholesale & retail | 2023 |
| Omniview Limited | Extraterritorial | 2018 |
| ON Demand Developments Ltd | Construction | 2024 |
| ON TAP Heating Services Limited | Construction | 2009 |
| One & Only Salon Ltd | Other services | 2023 |
| One Four Performance Ltd | Wholesale & retail | 2025 |
| One Planet Consulting Ltd | Professional & scientific | 2020 |