He belonged to the radical extremists influenced by Shinshichi ARIMA, and others in the same domain, but they were suppressed by the party advocating the integration of the imperial court and the shogunate in the Teradaya Incident of 1862, and thereby Oyama was suspended under house arrest in Satsuma.
同藩の有馬新七等に影響されて過激派に属したが、文久2年（1862年）の寺田屋事件では公武合体派によって鎮圧され、大山は帰国謹慎処分となる。