Probably O'Bannon's biggest contribution to science-fiction movies is his work on the screenplay of Alien, which started life as his script Star Beast. According to Empire Magazine, O'Bannon's script was the movie's first draft, although others later worked on the screenplay. And Empire says he brought over several of his colleagues from Alejandro Jodorowsky's abortive film Dune to Alien.
But prior to Alien, O'Bannon was co-writer and visual effects supervisor on John Carpenter's loopy Dark Star. And he also played Sgt. Pinback, as seen in the clip above. And O'Bannon worked on the original Star Wars, helping to craft those great computer graphics of the Death Star plans and the attack run — and according to this poster at IMDB, he's also in the movie, as one of the technicians in the Rebel Base during the Battle of Yavin.
After Alien, he wrote the great helicopter movie Blue Thunder plus two episodes of the spin-off TV series, and he wrote Lifeforce, "one of the movies that I still make people sit down and watch against their will," according to novelist Richard Kadrey on Twitter. O'Bannon also wrote two Philip K. Dick adaptations, Screamers and Total Recall. He wrote and directed the Romero-inspired zombie comedy, Return Of The Living Dead. And he worked on both the Heavy Metal comic as well as the movie, writing the "Soft Landing" and "B17" segments of the film. Some claim his work on the Heavy Metal comic influenced the visual style of Blade Runner.