• O'Brien Fine Foods
• O'Briens Wines & Beer
• O'Callaghan Collection
• O'Donoghue Ring Collection
• O'Leary International
• OASIS Group
• OCC Computer Personnel
• OCS Ireland

• OCS Recruitment
• ODEON Cinemas Holdings Limited
• Occupli
• Odyssey Social Care
• Office Buddies
• Office Talent
• Oliver James
• Oliver St. John Gogarty

• Optical Express
• Optimize Recruitment
• Oradeo
• Orange Recruitment
• Orchard Care Group
• Oriel House Hotel
• Orpea Residences Ireland
• Orwell Healthcare

• Osborne Recruitment
• Osprey Hotel
• Our Lady's Children's Hospital
• Oxfam Ireland
• Oxigen Environmental