Onishi was born in Edogawa, Tokyo. He graduated from Kokugakuin University and worked as an assistant to Diet members Takashi Fukaya and Ichiro Shimamura before entering politics in 1975 as a member of the Edogawa city assembly. He served as chairman of the assembly from 1984 to 1993, and then in the Tokyo metropolitan assembly from 1993 to 2007. He was a proportional representation candidate in the 2007 Japanese House of Councillors election but failed to win a seat. He remained active within the LDP's administrative ranks and was elected to the House of Representatives in the 2012 general election.[1]
In 2015 LDP leadership criticized Onishi for his suggestion that businesses should stop advertising in press outlets that criticize the government.[2] In 2017 Onishi drew media attention for controversial comments about cancer patients during a discussion of secondhand smoking regulations.[3]
