| O'Doherty Dental Care Limited | Health & social care | 2005 |
| Oasis Leisure Ltd. | Wholesale & retail | 2016 |
| OBL Limited | Professional & scientific | 2025 |
| OBL Projects Ltd | Construction | 2025 |
| OBO Dental Care Limited | Health & social care | 2018 |
| OBO Group Limited | Professional & scientific | 2018 |
| OBO Implantology Limited | Health & social care | 2018 |
| Odin Design Ltd | Manufacturing | 2020 |
| Ogbobi Consulting Ltd | Health & social care | 2019 |
| Ohatia Solutions Ltd | Information & communication | 2015 |
| OHS Manufacturing Limited | Manufacturing | 2025 |
| Okto Air Ltd | Professional & scientific | 2020 |
| Okto Integrated Services Ltd | Professional & scientific | 2022 |
| Okto Technologies Ltd | Professional & scientific | 2021 |
| Old Friends Day Care Ltd | Health & social care | 2023 |
| Old Pine Studios Ltd | Information & communication | 2021 |
| Old Wave Solutions Ltd | Hospitality | 2026 |
| Oliban Limited | Construction | 2016 |
| Olive Nora Ltd | Manufacturing | 2023 |
| Omagh BES Limited | Energy | 2023 |
| Omegasys Limited | Information & communication | 2009 |
| Omniplex (Banbridge) Limited | Arts & recreation | 2013 |
| Omniplex (Castlecourt) Limited | Arts & recreation | 2021 |
| Omniplex Holdings (NI) Limited | Information & communication | 1998 |
| Omnitech IT Ltd | Information & communication | 2021 |
| ON Road Distribution LLP | | 2018 |
| ON Track Technicians Ltd | Construction | 2016 |
| One New Enterprise Limited | Professional & scientific | 2006 |
| One New Enterprise Properties Limited | Construction | 2026 |
| One Office Supplies Limited | Wholesale & retail | 2021 |
| One Roar and Timbers Limited | Construction | 2018 |
| Onfold Ltd | Professional & scientific | 2024 |
| Online Info Solutions LLP | | 2017 |
| Online Media Management LLP | | 2017 |
| Onpoint Design Studio Ltd | Professional & scientific | 2021 |
| Onpoint Print & Display Ltd | Manufacturing | 2016 |
| Openview Consultancy International Limited | Information & communication | 1999 |
| OPT Systems Ltd | Wholesale & retail | 2026 |
| Optimum International Group LLP | | 2019 |
| Oran Enterprises Limited | Professional & scientific | 2003 |
| Orba Ltd | Manufacturing | 2021 |
| Orderway Ltd | Wholesale & retail | 2026 |
| Orica Blast & Quarry Surveys Limited | Mining & quarrying | 2007 |
| Origin Software Solutions Ltd | Information & communication | 2025 |
| Orion Capital & Investment Services Ltd | Professional & scientific | 2018 |
| Orion Distribution Limited | Wholesale & retail | 2023 |
| Orthocare Health Ltd | Health & social care | 2023 |
| OS Property Management Ltd | Real estate | 2023 |
| Osbourne Developments Limited | Construction | 2004 |
| Otahco Ltd | Real estate | 2024 |
| Otecc Consultancy Limited | Professional & scientific | 2023 |
| Otecc Limited | Real estate | 2016 |
| OTT Enterprises Ltd | Financial services | 2020 |
| Outdoor Hub Limited | Wholesale & retail | 2022 |
| Outpost Group Limited | Information & communication | 2018 |
| Outset Fitness Limited | Arts & recreation | 2025 |
| Outsider Games Ltd | Information & communication | 2012 |
| Oxford Gents Barbers Ltd | Other services | 2021 |
| Oyol C.I.C. | Information & communication | 2020 |
| P & E Property (N.I.) Ltd | Real estate | 2021 |
| P & E Solutions (NI) Ltd | Construction | 2012 |
| P & L Electrics Limited | Construction | 1996 |
| P Boylan Limited | Professional & scientific | 2023 |
| P C Electrical (2016) Ltd | Construction | 2016 |
| P J Brogan Ltd | Wholesale & retail | 2013 |
| P J Skips Limited | Water & waste | 2005 |
| P.D.W. Replacements Limited | Manufacturing | 1989 |
| P.F Engineering Services(Ni) Limited | Wholesale & retail | 2023 |
| PA Serv Ltd | Administrative & support | 2014 |
| Palaia Properties Limited | Real estate | 2013 |
| Palisade Technologies Ltd | Information & communication | 2023 |
| Paragon Street ( Lurgan) Limited | Arts & recreation | 2015 |
| Paragon Street Limited | Extraterritorial | 2015 |
| Parker Real Estate Holdings Limited | Professional & scientific | 2020 |
| Part 3 Consulting Limited | Information & communication | 2020 |
| Partcon Ltd | Construction | 2001 |
| PASHA07 Limited | Hospitality | 2025 |
| Patchwork Sounds Ltd. | Wholesale & retail | 2025 |
| Patricia Donnelly Consultancy Ltd | Professional & scientific | 2022 |
| Patterson Business Ltd | Wholesale & retail | 2009 |
| Patterson Consulting Ltd | Health & social care | 2018 |
| Patterson Haulage Ltd | Transport & logistics | 2023 |
| Patterson OIL Ltd | Wholesale & retail | 2022 |
| Patterson Outdoor Cleaning Solutions Limited | Administrative & support | 2003 |
| Patterson Retail Ltd | Wholesale & retail | 2009 |
| Paul Haffey Design Limited | Professional & scientific | 2004 |
| Paul Hampton Home Ltd | Wholesale & retail | 2015 |
| Paul Thompson Freight Limited | Transport & logistics | 2004 |
| Paws Doggy Daycare Ireland Ltd | Other services | 2026 |
| PBL Enterprises Ltd | Hospitality | 2019 |
| PC Elements Limited | Wholesale & retail | 2020 |
| PC Transport Ltd | Transport & logistics | 2013 |
| PCR Business Solutions Ltd | Wholesale & retail | 2016 |
| PCZ Properties Ltd | Real estate | 2021 |
| PD Corporation Limited | Manufacturing | 2011 |
| Pdkc Ltd | Information & communication | 2025 |
| Peacock Asset Management Limited | Real estate | 2015 |
| Peacock Investment Management | Financial services | 2024 |
| Peacock Investments | Financial services | 2024 |
| Peak Scaffolding Limited | Construction | 2025 |