Businesses
- O and R Fry, general commission agents
- O'Brien and Company
- O'Brien and Ireland's Repetoire Company
- O'Brien's, costumer
- O'Dea and Company
- O'Gorman and Ryan, draper
- O'Hagan Brothers
- O'Leary and Company
- O'Leary's Hotel
- O'Leary's, newsagents and booksellers
- O'Mara Limited
- O'Moore and Company
- O'Neill and Collins, solicitors
- O'Neill and Dwyer, grocers and provision merchants
- O'Shaughnessy and Gannon, merchant tailor
- O'Toole and Company
- Ormond Hotel
- Ormond Quay Presbyterian Endowments