• O'Brien Fine Foods
• O'Briens Wines & Beer
• O'Callaghan Collection
• O'Carroll Engineering
• O'Connor Sutton Cronin
• O'Leary International
• O'Toole Transport
• OASIS Group
• OCC Computer Personnel
• OCS Ireland
• ODEON Cinemas Holdings Limited
• OIC Cleaning
• ORS - Engineering Consultants

• Oak Fire Pizza
• Oaklodge Nursing Home
• Oakpark Foods
• Obelisk
• Ocean Sands Hotel
• Office Buddies
• Office Talent
• Oliver Bernard
• Oliver James
• Oliver St. John Gogarty
• Once Upon A Time Nursery and Montessori School
• OnceHub
• Onsite Group

• Open Text Corporation
• Optical Express
• Optima Recruitment
• Optimize Recruitment
• Optum
• Opus Recruitment Solutions
• Oracle Corporation
• Oradeo
• Orange Recruitment
• Oranmore Lodge Hotel
• Oriel House Hotel
• Origin Multilingual Recruitment
• Orion Group

• Ornua Food
• Orona
• Orpea Group
• Orwell Healthcare
• Osborne Recruitment
• Osprey Hotel
• Our Lady's Children's Hospital
• Outsource Support
• Overstock.com, Inc.
• Oxfam Ireland
• Oxford Global Resources
• Oxigen Environmental
• O’Neill & Brennan