She is especially well known for her songs from Revolutionary Girl Utena, Tales of Eternia The Animation, Slayers, Sorcerer Hunters, Di Gi Charat, Magic User's Club, Jungle de Ikou!, Akihabara Dennou Gumi, Ray the Animation, He Is My Master and Yu-Gi-Oh!

She has performed material for more than 50 singles and 20 albums to date (including her work with JAM Project and Chihiro Yonekura).

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 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.