| Oakshore Property Limited | Real estate | 2023 |
| Oaktree Wood Moulding Ltd | Manufacturing | 2013 |
| Objectivetop Ltd | Information & communication | 2020 |
| OCC Services Ltd | Construction | 2016 |
| OCF Solutions Ltd | Construction | 2015 |
| Odhran Limited | Information & communication | 2010 |
| Odisi Ltd | Wholesale & retail | 2019 |
| OF Estates Limited | Real estate | 2017 |
| Offsite Bimcraft Ltd | Professional & scientific | 2025 |
| Ohagan Ltd | Wholesale & retail | 2026 |
| Ohma Electrical Solutions Ltd | Construction | 2025 |
| Old Castle Developments Ltd | Construction | 2014 |
| Old Charm Inn Ltd | Hospitality | 2013 |
| Old Forge Contracting Limited | Construction | 2026 |
| Old Mill Collective Ltd | Real estate | 2025 |
| OMA GAS Ltd | Wholesale & retail | 2020 |
| Omagh Aluminium Systems Ltd | Manufacturing | 2003 |
| Omagh Child Contact Centre | Health & social care | 2016 |
| Omagh Chiropractic Ltd | Health & social care | 2010 |
| Omagh Craft Collective C.I.C. | Arts & recreation | 2012 |
| Omagh Early Years Centre | Health & social care | 1999 |
| Omagh Forum for Rural Associations | Other services | 2002 |
| Omagh Green Energy Construction Limited | Construction | 2026 |
| Omagh Group Pharmacy Limited | Health & social care | 1971 |
| Omagh Integrated Primary School Ltd | Education | 1989 |
| Omagh Lawn Tennis Club Limited | Arts & recreation | 2025 |
| Omagh Meats Limited | Manufacturing | 2017 |
| Omagh Motor Club Limited | Arts & recreation | 1954 |
| Omagh Motors Ltd | Wholesale & retail | 2019 |
| Omagh Music Supplies Ltd | Wholesale & retail | 2023 |
| Omagh Nail and Spa Ltd | Other services | 2023 |
| Omagh Plant Hire & Sales Ltd | Administrative & support | 2025 |
| Omagh RDA Limited | Other services | 2017 |
| Omagh Sand and Gravel Ltd | Mining & quarrying | 2025 |
| Omagh Specsavers Limited | Wholesale & retail | 2006 |
| Omagh Visionplus Limited | Wholesale & retail | 2006 |
| Omagh Volunteer Centre | Health & social care | 2001 |
| Omagh Women's AID | Other services | 2016 |
| Omagh Youth & Community Trust | Professional & scientific | 2000 |
| Omapharm Limited | Wholesale & retail | 2017 |
| Omasource Limited | Wholesale & retail | 2024 |
| Omatile LLP | | 2011 |
| Omatrade Limited | Manufacturing | 2017 |
| OMD Fabrications Limited | Manufacturing | 2018 |
| Omed Developments Ltd | Wholesale & retail | 2005 |
| Omega Crushing & Screening Holdings Limited | Financial services | 2024 |
| Omega Crushing and Screening Limited | Manufacturing | 2019 |
| One Percent Gym Ltd | Arts & recreation | 2025 |
| Online Support Europe Ltd | Information & communication | 2011 |
| ONP Ltd | Real estate | 2023 |
| Onyx Homes (Ireland) Ltd | Administrative & support | 2017 |
| Onyx Wellness Ltd | Arts & recreation | 2024 |
| Opex (IRE) Ltd | Professional & scientific | 2023 |
| Optec Ltd | Information & communication | 2018 |
| Optigear Ltd | Manufacturing | 2022 |
| Oran Oak Engineering Ltd | Construction | 2013 |
| Organic Beauty Goddess Ltd | Other services | 2025 |
| Organised Chaos Productions Limited | Information & communication | 2019 |
| Orig Ltd | Information & communication | 2023 |
| Origin Fresh Limited | Wholesale & retail | 2019 |
| Orthopaedic and Spine Solutions Limited | Wholesale & retail | 2024 |
| Orthos Medical Limited | Wholesale & retail | 2021 |
| Orthosurg Ltd | Health & social care | 2025 |
| Otown Property Group Ltd | Financial services | 2021 |
| Oughterard Contracts Ltd | Construction | 2014 |
| Outstanding Home Centre Ltd | Wholesale & retail | 2018 |
| Outstanding Property Solutions Ltd | Households | 2020 |
| Owen Conroy Ltd | Manufacturing | 2012 |
| Owenbrack Medical Limited | Health & social care | 2023 |
| Owenkillew Development Company Limited | Hospitality | 1997 |
| Oxgen Limited | Energy | 2014 |
| Oxtown Developments Ltd | Construction | 2025 |
| Ozone Games Limited | Arts & recreation | 2021 |
| Ozone Gaming Limited | Arts & recreation | 2012 |
| OZR Engineering Ltd | Manufacturing | 2025 |
| OZR Pubs Ltd | Hospitality | 2021 |
| OZR Trading Ltd | Wholesale & retail | 2022 |
| P & G Mcnulty Engineering Ltd | Manufacturing | 2010 |
| P Conway Transport Ltd | Transport & logistics | 2021 |
| P Donnelly Contracts Ltd | Construction | 2026 |
| P Gallagher & SON Limited | Transport & logistics | 2021 |
| P Quigley and SON Limited | Water & waste | 2015 |
| P&M Excavations Limited | Mining & quarrying | 2020 |
| P.K Top Construction NI Trading AS Kelly Recycling Limited | Construction | 2020 |
| P.R.O. Hire Centre (Derry) Limited | Administrative & support | 1988 |
| Pacino's Pizza House Ltd | Hospitality | 2022 |
| Pacino's Sweet Treats Limited | Hospitality | 2026 |
| Padon Limited | Construction | 2014 |
| Pampered and Precious Ltd | Wholesale & retail | 2026 |
| Parador Lodge Limited | Real estate | 2017 |
| Paragon Production Solutions Ltd | Manufacturing | 2020 |
| Parcmore Limited | Manufacturing | 2014 |
| Parkdenton Limited | Construction | 2006 |
| Party Town Marquees Ltd | Arts & recreation | 2015 |
| PAT Begley Joinery Ltd | Manufacturing | 2026 |
| Patrick Mccaul Environmental Consulting Engineers Limited | Professional & scientific | 2008 |
| Paul's Plant Hire Ltd | Administrative & support | 2011 |
| Pauline Mckenna Solicitors | Professional & scientific | 2016 |
| PD Coaches Limited | Transport & logistics | 2024 |
| Peadar Holdings Ltd | Financial services | 2019 |