| OCO Home Improvements Ltd | Real estate | 2025 |
| OCS Retail Ltd | Wholesale & retail | 2015 |
| Octopus Revenue Ltd. | Financial services | 2014 |
| Odeon Supplier Ltd | Wholesale & retail | 2023 |
| OG Waste Management Ltd | Wholesale & retail | 2023 |
| OGI Groundwater Specialists Limited | Professional & scientific | 1989 |
| OH Hello Gifts Ltd | Wholesale & retail | 2023 |
| Ohana MUA Limited | Financial services | 2024 |
| Ohana MUA Operations Ltd | Professional & scientific | 2024 |
| OIL & GAS Ltd | Mining & quarrying | 2019 |
| OIL Inspection Service Ltd | Other services | 2025 |
| OJ Hospitality Ltd | Hospitality | 2025 |
| OJM Tree & Landscape Services Ltd | Agriculture & fishing | 2009 |
| Okko Property Management Ltd | Real estate | 2025 |
| Okko Property Renovations Limited | Real estate | 2025 |
| Old Cinema Launderette Limited | Other services | 2011 |
| Old Elvet Limited | Manufacturing | 2019 |
| Old Elvet Living Limited | Real estate | 2022 |
| Old Tom's Company Limited | Hospitality | 2016 |
| Oldland Shopfitters Ltd | Construction | 2005 |
| Oliver Bainbridge Ltd | Wholesale & retail | 2009 |
| Omarandson Paving and Construction Ltd | Construction | 2025 |
| Omega TM Ltd | Construction | 2024 |
| Omnicom Solutions Holdings Limited | Information & communication | 2024 |
| Omnicom Solutions Limited | Information & communication | 2006 |
| Omnisave Solutions Ltd | Professional & scientific | 2023 |
| Omnus UK Limited | Professional & scientific | 2025 |
| One Collective Entertainment Ltd | Professional & scientific | 2026 |
| One Stop Vape Shop Ltd | Other services | 2018 |
| One UP Bistro Ltd | Hospitality | 2023 |
| Onebox Store Ltd | Wholesale & retail | 2025 |
| Onekothree Technologies Ltd | Information & communication | 2025 |
| Onetouch Servicez Ltd | Transport & logistics | 2020 |
| Onit Group Ltd | Construction | 2025 |
| Onpoint Measuring Ltd | Professional & scientific | 2020 |
| ONR Cosmetics Ltd | Wholesale & retail | 2024 |
| Openwex Ltd | Wholesale & retail | 2025 |
| Opportunist Limited | Wholesale & retail | 2024 |
| Opportunitee Byob Limited | Information & communication | 2026 |
| OPTIV8 Limited | Information & communication | 2019 |
| Opus CNC Limited | Wholesale & retail | 2017 |
| OR Property Solutions Limited | Real estate | 2026 |
| ORA Property & Development Ltd | Hospitality | 2022 |
| Orange PIP Properties Ltd | Real estate | 2018 |
| Orbina Ltd | Wholesale & retail | 2025 |
| Orca Cleaning Services Limited | Administrative & support | 2016 |
| Orchard Care (North East) Limited | Health & social care | 2005 |
| Orchard Garden (London) Limited | Extraterritorial | 2026 |
| Orchard Green Holdings Ltd | Financial services | 2020 |
| Orchard Green Land & Property Ltd | Real estate | 2024 |
| Orchard Thrive CIC | Education | 2026 |
| Oregon Estates Limited | Real estate | 2013 |
| Orenda Global Ltd. | Wholesale & retail | 2025 |
| Orfin 1 Limited | Financial services | 2024 |
| Orfin 2 Limited | Financial services | 2024 |
| Orfin 3 Limited | Financial services | 2024 |
| Origin Bathrooms Ltd | Construction | 2026 |
| Origo Fundraising CIC | Professional & scientific | 2024 |
| Orix Ltd | Wholesale & retail | 2024 |
| Orix Supplies Ltd | Manufacturing | 2025 |
| Orkids Training Hub Limited | Education | 2024 |
| ORT Global Limited | Wholesale & retail | 2024 |
| Orton Commercials Limited | Wholesale & retail | 2026 |
| Osborne Business Centre Ltd | Real estate | 2017 |
| OSG Contracts Limited | Construction | 2021 |
| Osman Dolek Ltd | Wholesale & retail | 2025 |
| Osteopathy and Therapy Northeast Limited | Health & social care | 2020 |
| Oswald Properties Ltd | Real estate | 2021 |
| OTS Estates Limited | Real estate | 2023 |
| OTS Heating Services Limited | Construction | 2013 |
| Ottopro Ltd | Construction | 2023 |
| OUR Auction Limited | Wholesale & retail | 2017 |
| OUR House North East Ltd | Real estate | 2022 |
| OUT for Delivery Ltd | Transport & logistics | 2023 |
| Outbound Logistics UK Limited | Professional & scientific | 2008 |
| Outdoor and Sustainability Education Specialists (Oases) Limited | Education | 1994 |
| Outdoor Design Logistics Limited | Manufacturing | 1999 |
| Outdoor Living Solutions Ltd | Construction | 2025 |
| Outside Living North East Ltd | Manufacturing | 2021 |
| Owengate Properties Limited | Real estate | 2024 |
| OX Close Farm Limited | Manufacturing | 2020 |
| OX International Ltd | Professional & scientific | 2019 |
| Ozbekler Ltd | Hospitality | 2025 |
| P & C Properties North East Limited | Real estate | 2016 |
| P & E.J. Simpson Limited | Agriculture & fishing | 2002 |
| P & K Events Ltd | Hospitality | 2025 |
| P & K Farmshop Ltd | Wholesale & retail | 2025 |
| P & P Catering Ltd | Hospitality | 2023 |
| P J Rushforth Limited | Transport & logistics | 2009 |
| P Michel Roofing Ltd | Construction | 2017 |
| P Morris Construction Ltd | Construction | 2024 |
| P&M Functional Fitness LLP | | 2022 |
| P.C. Henderson Limited | Manufacturing | 1974 |
| P.Jacksonservice Ltd | Professional & scientific | 2023 |
| P3 Technologies Ltd | Information & communication | 2026 |
| PAB Finance Ltd | Financial services | 2020 |
| Pacific Belgravia Limited | Other services | 2015 |
| PAJ Groundworks Limited | Construction | 2026 |
| Palatinate Properties Ltd | Real estate | 2025 |
| Palatine Trade Ltd | Wholesale & retail | 2025 |