Authors
- Patrick O'Brian 3
- Edna O'Brien 1
- Michael O'Brien 1
- M.J. O'Carroll 1
- Eileen O'Casey 1
- Daniel O'Connor 1
- Mary Ellen O'Connor 1
- Raymond J. O'Connor 1
- Joseph O'Conor 1
- George O'Dell 1
- Roy O'Dell 3
- Sean O'Donnell 1
- Kevin O'Donoghue 1
- Catherine O'Driscoll 2
- Eileen O'Faolain 2
- K.T. O'Grady 1
- Redmond O'Hanlon 3
- C.W. O'Hara 2
- Mary O'Hara 1
- Vincent P. O'Hara 1
- Colette O'Hare 2
- Cleophas C. O'Harra 1
- Pixie O'Harris 1
- Adelaide O'Keeffe 3
- Eamonn O'Keeffe 1
- Paul O'Keeffe 1
- Michael O'Leary 4
- Brian D. O'Neill 1
- Eamonn O'Neill 1
- Joan O'Neill 3
- John P. O'Neill 1
- Moira O'Neill 1
- Pat O'Reilly 1
- Tom O'Reilly 1
- G.G. O'Rourke 1
- Joseph O'Rourke 1
- Richard O'Shea 1
- Patrick O'Sullivan 2
- Tyrone O'Sullivan 1
- E.M. Oakeley 1
- E.R. Oakley 2
- Graham Oakley 14
- Michael Oakley 1
- J.A. Oates 1
- Ralf Obergfell 1
- Ulrich Oberprieler 2
- M.M. Oblinger 1
- Martine Oborne 1
- Waclaw Odyniec 1
- Christopher Ogborne 1
- Angela Ogden 3
- James Ogden 1
- J. Douglas Ogilby 1
- Malcolm A. Ogilvie 2
- Will H. Ogilvie 4
- David Ogilvy 1
- James S. Ogilvy 1
- Richard Ogle 1
- Arthur Oglesby 2
- Bob Ogley 3
- Hideaki Ohba 1
- Jane Ohlmeyer 1
- George C. Oke 1
- Thomas Okey 1
- Charles S. Olcott 1
- Frances Jenkins Olcott 1
- Jenny Oldfield 1
- Paul Oldfield 2
- Kenneth Oldham 1
- Tony Oldham 1
- Katharine Oldmeadow 6
- Michel Oleffe 1
- Achille Bonito Oliva 1
- W.H.T. Olive 1
- D. Oliver 1
- David Oliver 3
- Earl D. Oliver 1
- Jennie A. Oliver 1
- Jocelyn Oliver 1
- Kingsley M. Oliver 1
- Michael Oliver 1
- Thomas Oliver 1
- Torfrida Oliver 1
- Oliver W.R.B. 1
- Henry Olivier 1
- Jean Moulin Ollagnier 1
- John M. Olman 1
- Morton W. Olman 1
- Don Olney 1
- Nicholas Olsberg 1
- Frederick L. Olsen 1
- John Andreas Olsen 1
- Klaus Malling Olsen 1
- Lars-Henrik Olsen 1
- Gary D. Olson 1
- James S. Olson 1
- Lynne Olson 1
- Storrs L. Olson 1
- Stephen Olvey 1
- Frank Olynyk 1