| Nuco (Ireland) Limited | Energy | 2006 |
| Nugent Aviation Limited | Transport & logistics | 2016 |
| Nugent Coachworks Limited | Manufacturing | 2006 |
| Nugent Components Ltd | Wholesale & retail | 2020 |
| Nugent Contracts Ltd | Construction | 2024 |
| Nugent Fabrication Ltd | Extraterritorial | 2021 |
| Nugent Plant Limited | Wholesale & retail | 2024 |
| Nugent Poultry Ltd | Agriculture & fishing | 2015 |
| Nugent Trailers Limited | Manufacturing | 2023 |
| O & R Contracts Ltd | Construction | 2026 |
| O & S Doors Limited | Manufacturing | 1998 |
| O Harte UK Limited | Transport & logistics | 2020 |
| O Renascer Ltd | Hospitality | 2026 |
| O'Donnell Farms Ltd | Agriculture & fishing | 2015 |
| O'Hagan Electrics Limited | Construction | 2004 |
| O'Hanlon Electrical Limited | Energy | 2010 |
| O'Hanlon Murphy Electrical Limited | Construction | 2021 |
| O'Neill & Brady Contracts Limited | Construction | 2012 |
| O'Neill Brothers Brands Ltd | Wholesale & retail | 2024 |
| O'Neill Building Works Ltd | Construction | 2011 |
| O'Neill Construction(Ni) Ltd | Construction | 2020 |
| O'Neill Technology Recruitment Ltd | Information & communication | 2024 |
| O'Neill Vegetables Ltd | Agriculture & fishing | 2023 |
| O'Raifeartaigh Trans Ltd | Transport & logistics | 2023 |
| O'Reillys Sports (UK) Ltd | Wholesale & retail | 2025 |
| Oakleigh Homes Ltd | Financial services | 2026 |
| Oakpoint Estates Limited | Real estate | 2019 |
| Obsidotec Ltd | Other services | 2021 |
| OC Environmental Ltd | Professional & scientific | 2009 |
| OC International Transport Ltd | Transport & logistics | 2011 |
| Octocomms Limited | Professional & scientific | 2022 |
| OD Contracts & Crushing Ltd | Mining & quarrying | 2024 |
| Odcs Limited | Professional & scientific | 2002 |
| ODM Electrical Services Limited | Construction | 2016 |
| ODY Enterprises Ltd | Financial services | 2021 |
| OES Engineering Solutions Ltd | Manufacturing | 2012 |
| OFT Developments Limited | Real estate | 1994 |
| OGC Developments Ltd | Financial services | 2022 |
| Ogle Holdings Ltd | Financial services | 2023 |
| Ohagan Mcglinchey Ltd | Professional & scientific | 2019 |
| OHB Homes Ltd | Construction | 2025 |
| OK Property POT Ltd | Real estate | 2024 |
| OKB INS Services Ltd | Financial services | 2022 |
| Old Forest Lane Open Space Management Company Ltd | Households | 2025 |
| Olive Properties NI Limited | Professional & scientific | 2009 |
| Oliver Hagan & SON Ltd | Construction | 2019 |
| Olivia Muldoon Hair & Make UP Ltd | Other services | 2015 |
| Omac Oils Limited | Wholesale & retail | 2010 |
| OMC Creative Ltd | Professional & scientific | 2026 |
| OMC Plumbing Limited | Construction | 2015 |
| OMD Construction Ltd | Construction | 2020 |
| Omicode Ltd | Information & communication | 2025 |
| OMJ Utilities Limited | Construction | 2009 |
| OMJ Utility Infrastructure Services Ltd | Construction | 2019 |
| OMR Property Investment Limited | Real estate | 2023 |
| Oncor Holdings Limited | Professional & scientific | 2019 |
| Oncor Limited | Real estate | 2019 |
| ONE3TWO Aesthetics Ltd | Extraterritorial | 2022 |
| ONE3TWO Management Limited | Real estate | 2022 |
| Online Estate Agent N I Ltd | Real estate | 2016 |
| Online Sofas NI Ltd | Wholesale & retail | 2020 |
| Onls (Dungannon) Limited | Other services | 2023 |
| Onset Lighting & Rigging Ltd | Construction | 2020 |
| Onset Lighting Solutions Ltd | Information & communication | 2025 |
| OOD4 Ltd | Real estate | 2025 |
| OPOD6 Ltd | Real estate | 2025 |
| OPR Electrical Ltd | Construction | 2008 |
| Optimise E-Commerce Ltd | Wholesale & retail | 2024 |
| Optimus Control Systems Ltd | Manufacturing | 2019 |
| Orchard Country Limited | Extraterritorial | 1994 |
| Orchard County Contracts Ltd | Construction | 2010 |
| Orchard County Distribution Limited | Transport & logistics | 2025 |
| Orchard County Medical Ltd | Education | 2020 |
| Orchard Electrical Engineering Limited | Construction | 2024 |
| Orchard Hills Catering Limited | Manufacturing | 2023 |
| Orchard Plant Services Ltd | Wholesale & retail | 2014 |
| Order of Mary Servite Trust | Arts & recreation | 2020 |
| Orient Poultry (UK) Ltd | Wholesale & retail | 2019 |
| Orka Adventure Ltd | Wholesale & retail | 2024 |
| Osob Holdings Limited | Financial services | 2026 |
| Osood Holdings Limited | Financial services | 2026 |
| Ossian Commercial Funding Ltd | Financial services | 2025 |
| Ossian Commercial Ltd | Real estate | 2024 |
| Ossian Estates 2 Ltd | Real estate | 2026 |
| Ossian Estates Ltd | Real estate | 2025 |
| Ossian Residential Ltd | Construction | 2023 |
| Outback 365 Ltd | Wholesale & retail | 2017 |
| Outback Investments Limited | Real estate | 2014 |
| Outdoor Services (NOI) Ltd | Administrative & support | 2025 |
| Outlack Energy Ltd | Energy | 2012 |
| Outlook Press (N.I.) Limited | Information & communication | 1999 |
| Oval Agri Ltd | Wholesale & retail | 2025 |
| Oval Enterprises Ltd | Real estate | 2019 |
| Owens Furniture Group Ltd | Wholesale & retail | 2021 |
| P a Blevins & Co Ltd | Transport & logistics | 2018 |
| P Brannigan Ltd | Financial services | 2016 |
| P D Construction Ireland Limited | Construction | 2009 |
| P D Plumbing & Heating Ltd | Construction | 2002 |
| P G Chips Limited | Hospitality | 2014 |
| P Kane Engineering Ltd | Manufacturing | 2020 |