The Onslaught from Rigel
Fletcher Pratt
Oomphel in the Sky
H. Beam Piper
Federation (1)
Opening Atlantis
Harry Turtledove
Lost Continent of Atlantis (1)
Operation Earthworm
Joe Archibald
Operation Luna
Poul Anderson
Operation Chaos (2)
Operation Motherland
Scott Andrews
The Afterblight Chronicles (6)
Operation Sierra 75
Thomas S. Gressman
Vor - The Maelstrom (6)
Operation: Outer Space
Murray Leinster
An Opportunity for Profit
Dave Gross
Forgotten Realms: Double Diamond Triangle (5)
Opus 200
Isaac Asimov
Opus (2)
Oracles of Delphi Keep
Victoria Laurie
Oracles of Delphi Keep (1)
The Orb of Probability
Nat Schachner
Orca
Steven Brust
Vlad Taltos (7)
Origin
J. A. Konrath
Stephen Baxter
Manifold (3)
Orion
Ben Bova
Orion (1)
Orion Among the Stars
Orion (5)