| Oiltech Bearings Ltd | Manufacturing | 2018 |
| Oiski Poiski Limited | Information & communication | 2021 |
| OJ Jewellery Ltd | Wholesale & retail | 2020 |
| OJB Healthcare Ltd | Health & social care | 2022 |
| OJB Property Investments Ltd | Real estate | 2024 |
| OJC IT Ltd | Information & communication | 2016 |
| Ojemba Entertainment Limited | Arts & recreation | 2025 |
| OJM Services Ltd | Construction | 2021 |
| OJQ Management Ltd | Real estate | 2013 |
| OJZ IT Consultancy Ltd | Information & communication | 2019 |
| OJZ Properties Ltd | Real estate | 2020 |
| OK Please Limited | Hospitality | 2019 |
| Okana Global Limited | Professional & scientific | 2012 |
| Okehampton Court and Bodmin Court Management Company Limited | Real estate | 2000 |
| OKI Poke Ltd | Hospitality | 2022 |
| Okinsa International Ltd | Real estate | 2025 |
| Okkulo Limited | Arts & recreation | 2018 |
| Okunrin Meta Limited | Wholesale & retail | 2024 |
| Olaitan John Healthcare Ltd | Health & social care | 2023 |
| Olank Ltd | Hospitality | 2025 |
| Old School Gardens Residents Management Company Limited | Households | 2021 |
| Old Shanghai Cafe Limited | Extraterritorial | 2017 |
| Oldet Limited | Extraterritorial | 1959 |
| Oldnovo Rugby Ltd | Wholesale & retail | 2025 |
| Olds Technical Services Ltd | Professional & scientific | 2013 |
| OLI & Joes Newcastle Ltd | Hospitality | 2025 |
| OLI and Joes Gosforth Ltd | Hospitality | 2018 |
| OLI and Joes Limited | Hospitality | 2015 |
| Olias Bar Ltd | Hospitality | 2023 |
| Oliflo Engineering Services Ltd | Extraterritorial | 2017 |
| Oligra Holdings Limited | Real estate | 2024 |
| Oliphant (Newcastle) Ltd | Health & social care | 2022 |
| Olive Coffee Co Ltd | Hospitality | 2024 |
| Olive Compliance Ltd | Administrative & support | 2020 |
| Olive&Rose Holdings Limited | Real estate | 2023 |
| Oliver Financial Partnership Limited | Financial services | 2005 |
| Olivers Kitchen Ltd | Hospitality | 2022 |
| Olivia English Ltd | Real estate | 2025 |
| Olivia Learning Hub Ltd | Information & communication | 2026 |
| Olivia Mcclellan Ltd | Professional & scientific | 2025 |
| Olivia@Ch Limited | Other services | 2019 |
| Ollyblair Limited | Arts & recreation | 2005 |
| OLU Ventures Ltd | Real estate | 2026 |
| Olympia Gym NCL Ltd | Arts & recreation | 2021 |
| Olympius Developments 2 Limited | Construction | 2020 |
| Olympus Site Solutions NE Ltd | Construction | 2024 |
| Olympus Technology Limited | Wholesale & retail | 2026 |
| OM Developments (UK) Limited | Real estate | 2007 |
| OM Ganesha Building Ltd | Construction | 2025 |
| Omar Healthcare Limited | Health & social care | 2012 |
| Omar Najeeb Ltd | Other services | 2008 |
| Omexx Enterprises LP | | 2012 |
| Omicare Healthcare Ltd | Wholesale & retail | 2023 |
| Omie Ventures Ltd | Wholesale & retail | 2025 |
| Omni Construction Services Limited | Construction | 2019 |
| Omnia Consulting OFM Limited | Manufacturing | 2023 |
| Omnia Refrigeration Ltd | Professional & scientific | 2021 |
| Omnira Solutions Ltd | Information & communication | 2024 |
| Omnishield Technologies Limited | Manufacturing | 2026 |
| Ompay Ltd | Professional & scientific | 2023 |
| OMS Automotive Limited | Wholesale & retail | 2016 |
| OMS Partners Limited | Administrative & support | 2024 |
| ON Call Medical Limited | Health & social care | 2019 |
| ON Loan Limited | Professional & scientific | 2004 |
| ON Medical Limited | Health & social care | 2006 |
| ON OUR Doorstep Community Interest Company | Health & social care | 2022 |
| ON Point Joinery & Building Limited | Construction | 2015 |
| ON the Flipside Ltd | Wholesale & retail | 2026 |
| ON the Go Accessory Ltd | Wholesale & retail | 2020 |
| ON the Go Pizza Ltd | Hospitality | 2021 |
| ON the Go Services Ltd | Wholesale & retail | 2025 |
| ON the Quay Limited | Hospitality | 2018 |
| ON the UP (North-East) CIC | Education | 2019 |
| ON the Wing Limited | Administrative & support | 2025 |
| ON Track Retail Limited | Administrative & support | 2014 |
| On-Hire Limited | Administrative & support | 2000 |
| Onah Ltd | Wholesale & retail | 2026 |
| One (OWL North East) Trust | Education | 2023 |
| One Asset Management (NE) Limited | Administrative & support | 2023 |
| One Click Storage (Consett) Ltd | Other services | 2026 |
| One Click Storage Limited | Other services | 2024 |
| One Eldon Ltd | Real estate | 2025 |
| One Electrical NE Limited | Construction | 2026 |
| One Eleven Productions Limited | Wholesale & retail | 2025 |
| One Environments (Holdings) Ltd | Financial services | 2022 |
| One Environments Ltd | Professional & scientific | 2017 |
| One Environments Trustees Limited | Administrative & support | 2024 |
| One Financial Management Ltd | Administrative & support | 2025 |
| One Group (NE) Limited | Administrative & support | 2023 |
| One Lease (Stanley) Limited | Real estate | 2024 |
| One Lease (Villas) Limited | Real estate | 2024 |
| One Lease DS Limited | Real estate | 2023 |
| One Lease Eldon Ltd | Real estate | 2024 |
| One Lease Management (NCL) Limited | Real estate | 2023 |
| One Lease Management (NE) Ltd | Real estate | 2023 |
| One Lease Management Limited | Administrative & support | 2023 |
| One Lease River Ltd | Real estate | 2024 |
| One Lease Seaham Limited | Real estate | 2024 |
| One Management (NE) Ltd | Administrative & support | 2023 |
| One Million Women and Girls Ltd | Information & communication | 2025 |