| Nwtr Limited | Wholesale & retail | 2011 |
| O Donnell Assessors Ltd | Construction | 2025 |
| O K Cranes & Formwork Limited | Construction | 2005 |
| O K I Developments Limited | Construction | 2005 |
| O'Connor Burke Architecture Ltd | Professional & scientific | 2015 |
| O'Connor Transport Derry Limited | Transport & logistics | 2015 |
| O'Donnell Properties (Contracts) Ltd | Construction | 2010 |
| O'Donnell Properties Limited | Construction | 2006 |
| O'Hagan Donaghy Property Ltd | Real estate | 2016 |
| O'Kane Contracts Limited | Construction | 2022 |
| O'Kane Meats (NI) Ltd | Wholesale & retail | 2018 |
| O'Kane Upholstery Limited | Other services | 2020 |
| O'Kane Woodworking 2010 Limited | Construction | 2010 |
| O'Neill Bros. (Building Contractors) Limited | Wholesale & retail | 1988 |
| O'Neill Proweld Limited | Construction | 2015 |
| O2B Happy Happy Limited | Extraterritorial | 2015 |
| OAC Properties Ltd | Real estate | 2025 |
| Oak & Glen Properties Ltd | Real estate | 2025 |
| Oak and Steel Exchange Ltd | Wholesale & retail | 2025 |
| Oak City Holdings Limited | Financial services | 2024 |
| Oak Leaf Properties Limited | Real estate | 1986 |
| Oak Lodge Finance Ltd | Wholesale & retail | 2026 |
| Oakdene Drinks Ltd | Hospitality | 2012 |
| Oakfire Adventures Ltd | Arts & recreation | 2010 |
| Oakgrove Integrated College Limited | Education | 2001 |
| Oakgrove Integrated Primary School Limited | Extraterritorial | 1991 |
| Oakland Recruitment Services Ltd | Administrative & support | 2025 |
| Oakleaf Analytics Ltd | Information & communication | 2025 |
| Oakleaf Coffee Co Ltd | Hospitality | 2026 |
| Oakling Educational Psychology Ltd | Education | 2022 |
| Oakmore Plastering Limited | Construction | 2020 |
| Oaks Lodge Limited | Real estate | 2017 |
| Oaktrees Psychological Services Ltd | Health & social care | 2024 |
| Oakvale Group Ltd | Construction | 2024 |
| OB Spray Rendering Limited | Construction | 2021 |
| Obel Ground Ltd | Real estate | 2018 |
| Obel Management Limited | Real estate | 2021 |
| Obel Tower Ltd | Real estate | 2018 |
| OBS Ireland NI Ltd | Professional & scientific | 2025 |
| Obsera AI Ltd | Information & communication | 2025 |
| OC Wealth Management Ltd | Financial services | 2024 |
| OCG Productions Ltd | Information & communication | 2017 |
| OCT Transport L Ltd. | Wholesale & retail | 2011 |
| Oculus Ireland Limited | Information & communication | 2024 |
| Odem Limited | Professional & scientific | 2015 |
| Oder Ltd | Real estate | 2023 |
| ODK Holdings Limited | Financial services | 2026 |
| ODP Projects Ltd | Construction | 2019 |
| OFE Ltd | Other services | 2024 |
| OFF Licence Corporation Limited | Wholesale & retail | 2026 |
| Offing Ltd | Hospitality | 2023 |
| Oghill Farm Woodwork Ltd | Manufacturing | 2025 |
| Ogilby Limited | Professional & scientific | 2001 |
| Ohagan Trading Ltd | Hospitality | 2019 |
| Ohara Investments Ltd | Real estate | 2025 |
| OHM Services Limited | Construction | 2024 |
| Oiwillo Training Co. Ltd. | Education | 2007 |
| OK Wellbeing Solutions Limited | Education | 2025 |
| Old Court Property Management Company Limited | Real estate | 2009 |
| Olinah Properties Ltd | Real estate | 2025 |
| Oliver Boat Trading Ltd | Wholesale & retail | 2025 |
| Oliver Clarke Property Ltd | Real estate | 2026 |
| Oliver Clarke Residential Ltd | Real estate | 2025 |
| Olympus Fitness Ltd | Wholesale & retail | 2016 |
| Omega Mechanical Services Limited | Construction | 1992 |
| Omega Property Investments Limited | Real estate | 2017 |
| Omerta Investments Limited | Real estate | 1995 |
| OMS Auto Parts Limited | Wholesale & retail | 2020 |
| ONC Begley Ltd | Wholesale & retail | 2024 |
| One Luxury Limited | Wholesale & retail | 2014 |
| Online Motors Area Ltd | Wholesale & retail | 2026 |
| Oocc Ltd | Wholesale & retail | 2021 |
| OP Buildworks Limited | Construction | 2025 |
| Open Padel Ltd | Arts & recreation | 2025 |
| Optik AI Limited | Information & communication | 2025 |
| OR Homes Ltd | Real estate | 2021 |
| Oracle Aesthetics Ltd | Health & social care | 2017 |
| Origin Enterprises UK Limited | Professional & scientific | 2005 |
| Original Heating Limited | Construction | 2012 |
| Ormeley Investments Limited | Real estate | 2016 |
| Orthomd Limited | Health & social care | 2023 |
| Oryx TCG Ltd | Wholesale & retail | 2025 |
| OUR Career MAP Ltd | Administrative & support | 2024 |
| OUR Streets Youth Ltd | Other services | 2022 |
| Outlaw and Ekuban Enterprises Limited | Wholesale & retail | 2025 |
| Outlet Furniture Ltd | Wholesale & retail | 2013 |
| Ovflo Group Limited | Extraterritorial | 2015 |
| Owenbeg Investments Ltd | Real estate | 2022 |
| Oxford Bulls FC | Other services | 2019 |
| Oxxy Limited | Extraterritorial | 2016 |
| P Fallon Electrical Services Ltd | Construction | 2025 |
| P J'S Derry Limited | Wholesale & retail | 2014 |
| P K Takeaway Limited | Hospitality | 2026 |
| P Lynch Plumbing & Heating Limited | Construction | 2022 |
| P Mullan Construction Ltd | Construction | 2024 |
| P Ward Homes Limited | Construction | 2016 |
| P&D Ennis Ltd | Wholesale & retail | 2024 |
| P. Dunne BUS Hire Ltd | Transport & logistics | 2018 |
| P. Mcnabb & Company Limited | Wholesale & retail | 1993 |
| P.C.E.A Limited | Hospitality | 2024 |