After graduation, Oomori moved to Tokyo and began his career at Studio Annapuru in 1981, a small sub-contracting studio founded by Osamu Dezaki and Akio Sugino. Soon after he left the studio and joined Bebow, a studio ran by Tomonori Kogawa. There he mainly worked on mecha shows produced by Sunrise. He also showed up as an animation director on Urusei Yatsura.
Oomori left Bebow in 1986, and participated in the establishment of Atelier Giga. Around this time he also served as an animation director on Mobile Suit Gundam: Char's Counterattack, one of his best-known works.
In the '90s he was also active in the video game industry. Notably as an animation director on Final Fantasy VII.
