Born in Osaka, Ōmori studied at Kyoto Prefectural University of Medicine and actually holds a license to practice medicine. While in school, he began making films independently, with Kuraku naru made matenai! (1975), which featured Seijun Suzuki, receiving particularly high praise. His script "Orenji rōdo kyūkō" won the 3rd Kido Award for screenplays in 1977, and the next year he was able to film that in his professional debut. Several of his films, such as the 1980 Hipokuratesu-tachi, feature doctors or rely on his knowledge of medicine. He has worked in a variety of genres, including suspense films, musicals, and most famously abroad, several contributions to the Heisei Godzilla series.