| O'Hanlon & Farrell Holdings Limited | Construction | 2007 |
| O'Hare & Grant Groundworks Ltd | Construction | 2026 |
| O'Hare & Mcguinness Ltd | Administrative & support | 2003 |
| O'Hare & Sons Contractors Limited | Construction | 2015 |
| O'Hare Contracts Ltd | Construction | 2019 |
| O'Hare Properties Ltd | Real estate | 2017 |
| O'Hare Steel Limited | Manufacturing | 2010 |
| O'Hares Drumaness Ltd | Wholesale & retail | 2014 |
| O'Hares Lurgan Limited | Financial services | 2022 |
| O'Hares Rathfriland Limited | Wholesale & retail | 2016 |
| O'Hares Supermarket Limited | Wholesale & retail | 2012 |
| O'Hares Warrenpoint Limited | Wholesale & retail | 2017 |
| O'Loan & Co. Ltd | Administrative & support | 2001 |
| O'Malley Transport Services Ltd | Transport & logistics | 2022 |
| O'Neill International Ltd | Transport & logistics | 2021 |
| O'Neills Bars 2025 Ltd | Hospitality | 2025 |
| O'Reilly Enterprises Limited | Professional & scientific | 2026 |
| O'Reilly's the Sweet People Limited | Wholesale & retail | 1989 |
| O'Reilly's Wholesale Limited | Wholesale & retail | 2010 |
| O. N Contracts Ltd | Construction | 2017 |
| O.Rooney Coach Hire Ltd. | Transport & logistics | 2016 |
| Oak Commercials (Newry) Ltd | Wholesale & retail | 2007 |
| Oak Grove Property Management Limited | Households | 2005 |
| Oak Tree Roofing and Carpentry Ltd | Construction | 2021 |
| Oakleaf Technology Ltd | Financial services | 2023 |
| Oakridge Agri Ltd | Wholesale & retail | 2019 |
| Oakridge Motors Ltd | Wholesale & retail | 2025 |
| Oathello Limited | Information & communication | 2022 |
| OBM Imports Ltd | Transport & logistics | 2025 |
| OC Livestock Ltd | Wholesale & retail | 2018 |
| OCB Developments Limited | Construction | 2012 |
| Occupational Therapy Northern Ireland (Otni) Ltd | Health & social care | 2021 |
| OCD Electrical Limited | Construction | 2006 |
| OCF Services Ltd | Professional & scientific | 2024 |
| Ococ Limited | Construction | 2019 |
| OCR Lurgan Ltd | Real estate | 2022 |
| Odda Group LP | | 2019 |
| ODM Contract Maintenance (NI) Ltd | Construction | 2010 |
| Odyssey Electrical Ireland Ltd | Construction | 2025 |
| OEM Consultancy Ltd | Professional & scientific | 2016 |
| Offshore Rotary Solutions Ltd | Professional & scientific | 2024 |
| OGC for Medicine and Healthcare Limited | Administrative & support | 2013 |
| OH Contracts Ltd | Construction | 2024 |
| OH Fitness Ltd | Arts & recreation | 2024 |
| OHK Energy (UK) Limited | Construction | 2024 |
| Ohmg (Holdings) Limited | Construction | 2016 |
| Ohmg Ltd | Construction | 2014 |
| Ohph Ltd | Construction | 2018 |
| Oisin Mcclelland Ltd | Wholesale & retail | 2025 |
| OJA Homes Ltd | Real estate | 2018 |
| OJM Touchstone Ltd | Professional & scientific | 2017 |
| OK Transport Ltd | Transport & logistics | 2023 |
| Okin Care Services Ltd | Administrative & support | 2025 |
| Old Church Court Kilkeel Management Company Limited | Financial services | 2024 |
| Old Fort Lodge Moyraverty Ltd | Households | 2011 |
| Old Mill Maghera (Management) Ltd | Real estate | 1999 |
| Old Mill Race Management Ltd | Households | 2016 |
| Old Newry Publications Limited | Information & communication | 2020 |
| Old Quarry Manor Management Limited | Households | 2026 |
| Old Railway View Management Limited | Real estate | 2004 |
| Old Station Dunadry Management Co Ltd | Households | 2004 |
| Oldberg Projects L.P. | | 2017 |
| Oldmark Universal L.P. | | 2017 |
| OM Construction Newry Ltd | Administrative & support | 2026 |
| Omcc Joinery Limited | Construction | 2016 |
| Omey Ltd | Education | 2011 |
| Omni Power Technologies Ltd | Construction | 2019 |
| Onpoint Digital Ltd | Information & communication | 2018 |
| Onvas Ltd | Information & communication | 2026 |
| Onyx Fitness Systems Limited | Professional & scientific | 2021 |
| Onyx Styling Limited | Professional & scientific | 2024 |
| Ooosch Giveaways Limited | Wholesale & retail | 2021 |
| OP International Services LP | | 2020 |
| Opale Corporation LP | | 2017 |
| Opcom Services Ltd | Wholesale & retail | 2016 |
| Opitec Shop Limited | Manufacturing | 2018 |
| Optiforce Limited | Information & communication | 2007 |
| Optimum Homecare NI Ltd | Health & social care | 2019 |
| ORA Consulting Limited | Professional & scientific | 2026 |
| Oranmac Limited | Hospitality | 2017 |
| Orby Engineering Limited | Manufacturing | 2016 |
| Orca Commerce Ltd | Wholesale & retail | 2025 |
| Orchard Distribution Ltd | Transport & logistics | 2015 |
| Orchard Software Consulting Ltd | Information & communication | 2013 |
| Orchard ST Leonards Limited-The | Administrative & support | 1995 |
| Orchard ST. Leonard's Management Company Limited - the | Extraterritorial | 1996 |
| Orchard Tyres Ltd | Wholesale & retail | 2023 |
| Oriel Orthodontics Ltd. | Health & social care | 2014 |
| Oriental Red Warrenpoint Ltd | Hospitality | 2023 |
| Orion GXP Consulting Ltd | Professional & scientific | 2023 |
| Orior by Design Limited | Manufacturing | 2000 |
| Orourke Contracts Limited | Construction | 2018 |
| Orso Distribution N.I Limited | Wholesale & retail | 2023 |
| OS Brick and Block Limited | Construction | 2025 |
| OSM (IRL) Ltd | Manufacturing | 2006 |
| OSM Trading UK Limited | Wholesale & retail | 2018 |
| Osstre Limited | Information & communication | 2025 |
| Oton (N.I.) Limited | Real estate | 2023 |
| Outdoo Limited | Wholesale & retail | 2024 |
| Overland Express Customs Clearance Limited | | 1993 |