The Writers Guild of America confirmed his death. The cause was Crohn's disease, a chronic gastrointestinal disorder that Mr. O'Bannon endured for 30 years, his wife, Diane, told The Los Angeles Times.

Mr. O'Bannon had an early start as a screenwriter when he and the director John Carpenter, students at the time at the University of Southern California film school, wrote the low-budget film Dark Star, which was released as a feature in 1974. After working as a computer animator for the director George Lucas on Star Wars and trying, unsuccessfully, to develop a film based on the Frank Herbert novel Dune, Mr. O'Bannon created the story of Alien with the screenwriter Ronald Shusett and wrote the screenplay on his own.

The film, directed by Ridley Scott and starring Sigourney Weaver, is about a spaceship with a vicious monster loose onboard. (The creature begins as a parasite that explodes from a crew member's chest.) It became a box office hit, a classic of science fiction and horror, and the progenitor of a lucrative Hollywood franchise, with its several sequels.

"I love gore films and I grew up with '50s monster movies," Mr. O'Bannon told the journal Cinefantastique in 1979, speaking of the film's origins. "The idea for the monster in Alien originally came from a stomach ache I had."