- Obnoxio the Clown vs the X-Men (1983)
- Offcastes (1993, Heavy Hitters)
- Official Handbook of the Conan Universe, Vol. 1 (1986)
- Official Handbook of the Conan Universe, Vol. 2 (1993)
- Official Handbook of the Marvel Universe, Vol. 1 (1981–1983) (aka Classic Edition)
- Official Handbook of the Marvel Universe, Vol. 2 (1985–1986) (aka Deluxe Edition)
- Official Handbook of the Marvel Universe, Vol. 3 (1981–1989) (aka Update '89)
- Official Handbook of the Marvel Universe, Vol. 3.5 (1990) (aka Master Edition)
- Official Handbook of the Marvel Universe, Vol. 4 (2004–2005) (aka 2004/2005 Edition)
- Official Handbook of the Marvel Universe, Vol. 5 (2007-current) (aka 2007 Edition)
- Official Handbook of the Marvel Universe A-Z (2008–2010)
- Official Handbook of the Marvel Universe A-Z Update (2010-current)
- Official Index to the Marvel Universe, Vol. 1 (2009–2010)
- Official Index to the Marvel Universe. Vol. 2 (2010-current)
- Official Marvel Index to the Amazing Spider-Man (1985)
- Official Marvel Index to the Fantastic Four (1985–1987)
- Official Marvel Index to Marvel Team-Up (1986–1987)
- Official Marvel Index to the Avengers, Vol. 1 (1987–1988)
- Official Marvel Index to the Avengers, Vol. 2 (1994–1995)
- Official Marvel Index to the X-Men, Vol. 1 (1987–1988)
- Official Marvel Index to the X-Men, Vol. 2 (1994)
- Official True Crime Cases (1947, Atlas Comics)
- Olympians (1991–1992)
- Omega Flight (2007)
- Omega: The Unknown (1976–1977)
- One, The (1985, Epic)
- Onslaught: Epilogue (1997)
- Onslaught: Marvel Universe (1996)
- Onslaught Reborn (2006-current)
- Onslaught: X-Men (1996)
- Open Space (1989–1999)
- Order, The, Vol. 1 (2002)
- Origin of Galactus (1996)
- Original Ghost Rider, The (1992–1994)
- Original Ghost Rider Rides Again, The (1991–1992)
- Origins of Siege (2009)
- Ororo: Before The Storm (2005)
- Oscar Comics (1947–1949, Atlas Comics)
- Our Love (1949–1950)
- Our Love Story (1969–1976)
- Outlaw Fighters (1954–1955, Atlas Comics)
- Outlaw Kid, Vol. 1 (1954–1957, Atlas Comics)
- Outlaw Kid, Vol. 2 (1970–1975)
- Over the Edge (1995–1996)
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