[4] Finally, Oikawa made her debut adult video at age nineteen for the Media Station Cosmos Plan label in September 2000.

In her first year of adult work, she made only a few videos, including a softcore V-Cinema movie, Virgin Teacher Hinako in September 2001.

[4][7] Her first video with KMP and Tameike, Another Side of Nao Oikawa, involved anal sex and forced fellatio.

Together with Ran Monbu, Saori Kamiya and Hitomi Hayasaka, she was part of the exclusive group of actresses that KMP used for promotion under the name "2003 Million Girls" (ミリオンガールズ2003, Miriongāruzu 2003).

[10] On a gentler note, Oikawa appeared with Kurumi Morishita in a June 2003 production by KMP entitled Forest in Nude.

Directed by TOHJIRO, this lesbian genre video is a "soft, bittersweet story of two friends who remember the days of their youth and the sexual experiences they had".

Oikawa, along with fellow AV actress Mariko Kawana, actor Taka Kato, director Goro Tameike and Soft On Demand founder Ganari Takahashi, was among the 17 people interviewed for Misato Nakayama's study of professionals in the adult industry, 性職者の人々 あの世界の仕事師たち (Sei Shoku Sha No Hitobito Ano Sekai No Shigotoshi Tachi), published in January 2006 by Ohzora (ISBN 4-7767-9229-X).

[16] On November 9, 2007 she made a guest appearance in the 6th episode of Tsuburaya Productions mini-series Ultraseven X.Oikawa was cast as Negi's advisor "Shizuna Minamoto" in the 2007-08 late-night TV Tokyo series Negima!

In the September 21, 2008 episode, Oikawa and her co-stars Rina Aizawa and Yumi Sugimoto teamed up as the one-off G3 Princess singing group,[18] releasing both an EP and a CD box set featuring the group's song G3 Princess Rap ~Pretty Love Limited~ and Oikawa's solo character song Utopia.

From November to December 2007 Oikawa performed her first lead stage role in a three-person drama called Night Mess.