Authors
- Raghnall O Floinn 1
- Patrick O'Brian 3
- Edna O'Brien 2
- Michael O'Brien 1
- M.J. O'Carroll 1
- Eileen O'Casey 1
- Daniel O'Connor 1
- Raymond J. O'Connor 1
- George O'Dell 1
- Roy O'Dell 3
- Kevin O'Donoghue 1
- Catherine O'Driscoll 2
- Eileen O'Faolain 2
- K.T. O'Grady 1
- Redmond O'Hanlon 1
- C.W. O'Hara 2
- Vincent P. O'Hara 1
- Colette O'Hare 2
- Cleophas C. O'Harra 1
- Finola O'Kane 1
- Adelaide O'Keeffe 3
- Eamonn O'Keeffe 1
- Paul O'Keeffe 1
- Michael O'Leary 4
- Brian D. O'Neill 1
- Joan O'Neill 3
- John P. O'Neill 1
- Moira O'Neill 1
- Robert O'Neill 1
- Pat O'Reilly 1
- Tom O'Reilly 1
- Joseph O'Rourke 1
- Tyrone O'Sullivan 1
- E.R. Oakley 1
- Graham Oakley 2
- Michael Oakley 1
- Mike Oakley 1
- J.A. Oates 1
- Jonathan Oates 1
- Ralf Obergfell 1
- Ulrich Oberprieler 1
- Waclaw Odyniec 1
- Richard Offner 1
- Christopher Ogborne 1
- Angela Ogden 3
- J. Douglas Ogilby 1
- Malcolm A. Ogilvie 2
- Will H. Ogilvie 6
- James S. Ogilvy 1
- Arthur Oglesby 2
- Bob Ogley 3
- Hideaki Ohba 1
- Jane Ohlmeyer 1
- Thomas Okey 1
- Charles S. Olcott 1
- Frances Jenkins Olcott 1
- Jenny Oldfield 1
- Paul Oldfield 1
- Kenneth Oldham 1
- Katharine Oldmeadow 5
- Achille Bonito Oliva 1
- Martin Olive 1
- W.H.T. Olive 1
- D. Oliver 1
- David Oliver 3
- Earl D. Oliver 1
- Gerald Oliver 1
- Jennie A. Oliver 1
- Jocelyn Oliver 1
- Michael Oliver 1
- Thomas Oliver 1
- Torfrida Oliver 1
- Oliver W.R.B. 1
- Henry Olivier 1
- Jean Moulin Ollagnier 1
- Robert P. Olmsted 1
- Don Olney 1
- Bjorn Olsen 1
- John Andreas Olsen 1
- Klaus Malling Olsen 3
- Penny Olsen 1
- Gary D. Olson 1
- James S. Olson 1
- Lynne Olson 1
- Storrs L. Olson 1
- Urban Olsson 1
- Charles Oman 2
- F.D. Ommanney 1
- Urgunge Onon 1
- William Hillier Onslow 1
- Willem Oosthoek 1
- Iona Opie 10
- Peter Opie 6
- Paul Oppenheimer 1
- Hiawyn Oram 1
- Vincent Orange 3
- Arthur W.J.G. Ord-Hume 1
- John Organ 1
- Doris Orgel 1
- Laszlo Orloci 1