Oyamada was born in Setagaya, Tokyo, Japan. His first claim to fame was as a member of the pop duo Flipper's Guitar, one of the key groups of the Tokyo Shibuya-kei scene. Following the disbandment of Flipper's Guitar in 1991, Oyamada donned the "Cornelius" moniker and embarked on a successful solo career. He chose his pseudonym in tribute to the character of the same name from the movie "Planet of the Apes". He commissioned a song, about himself, on Momus' 1999 album "Stars Forever".