Keigo Oyamada (小山田 圭吾 Oyamada Keigo, born January 27, 1969), also known by his moniker Cornelius, is a Japanese recording artist and producer who co-founded Flipper's Guitar, an influential Shibuya-kei band, and subsequently embarked on a solo career.

In 1997, he released the album Fantasma, which landed him praise from American music critics, who called him a "modern-day Brian Wilson" or the "Japanese Beck".