Okui began her music career as a concert backup singer for Yuki Saito in November 1989. Her first solo single "Dare Yori mo Zutto" was released in 1993, and was used as a theme song for the anime OAV The Girl from Phantasia. She sang with voice actress Megumi Hayashibara for the anime television series Slayers in 1995. In 2003, she teamed up with fellow artist Chihiro Yonekura to form the group r.o.r/s, an acronym for "Reflections of Renaissance/Sounds". She also became a member of the supergroup JAM Project, which released the single "Little Wing" that served as the opening theme song for the anime television series Scrapped Princess. She left Starchild Records and produced her own record label called "evolution", which lasted from 2003. In 2011, she ended her evolution label, left Geneon Universal Entertainment, and signed with Lantis.
Okui is a regular performer in the Animelo Summer Live annual concerts from "The Bridge" in 2005 to "Rainbow" in 2011. In October 2008, she went to Mexico with JAM Project where they performed at the Expo TNT 16. She also performed in 2012 for EXPO TNT GT 6.