Though his life outside of his job at the facility is unknown, one can presume he might have also attended Kamakura's university like Kurama or even worked there like Kakuzawa. During their time as coworkers, Kurama treated him with respect and concern, while Kakuzawa was shown to make fun of him on occasion.
During a routine day at the research institute, Silpelit #3 escaped during her feeding time and ran free through the halls in her search for Kurama, who she'd realized was the only human still looking at the Diclonius subjects as people at the time. She met Oomori outside their office first but didn't kill him as she'd done to the guard outside. She infected both men with the Diclonius Virus using her vectors, altering their DNA without their knowledge. Number Three was promptly killed by Kakuzawa, leaving Kurama to puzzle over what she meant by her wanting to "thank" him.
Not long after, both men learned their wives were going to give birth at around the same time, with Oomori excitedly saying their daughters would probably be classmates. Kakuzawa again taunted Oomori, this time with the idea that his daughter would be a horned girl, but at the time, no one knew how or why these little girls were being born. Oomori's wife gave birth a few weeks before Kurama's wife, only to find that his daughter was a Diclonius. Since Chief Kakuzawa had previously laid down a policy of euthanizing all Silpelit little girls born after they began to run out of room to house them all in the facility, Oomori begged Kurama and Professor Kakuzawa for his daughter's life. Though Kurama agreed to take the hateful decision to kill his daughter out of his hands, a heartbroken Oomori left the Research Institute entirely. Given the confidential nature of the research he had aided in and the paranoia of the Kakuzawas; it is possible to speculate that he was either kept under watch or left somehow incapable of telling others about the Institute's activities. His fate and torment foreshadowed Kurama's own with Mariko. It is unknown if he ever learned that Kurama made a deal to save his own child's life. While Isobe, in essence, took Oomori's place, it is also unknown whether his title, rank or position was the same, or precisely when after Oomori's departure he started.
Strangely, Oomori's name is spelled "Ohmoni" in official sources for the English-language dub.