Following Takechi, in September 1856 Okada went to Edo and studied Kyōshin Meichi-ryū [ja] at Shigakukan, which was the training hall of Momonoi Shunzō.
Takechi considered that house of Okada would have difficulties with covering travel expenses and so asked the feudal lord of Oka domain in Bungo province to accommodate his student.
The same year he joined Tosa Kinnoto, organized by Takechi and loyal to the Sonnō Jōi movement.
[1] Starting with Inoue Saichiro, who was the shita metsuke (low class inspector of foot soldiers) of the Tosa Domain, Izō assassinated Honma Seiichiro who was his comrade, Ikeuchi Daigaku, Mori Magoroku, Ogawara Juzo, Watanabe Kinzan, and Ueda Jonosuke, who were government officials and yoriki (police sergeants) that belonged to the Kyoto City Magistrate, Tada Tatewaki (son of Nagano Shuzen who commanded Ansei no Taigoku [suppression of extremists by the Shogunate] and Murayama Kazue, who during the assassination was tied to a bridge and made a public display alive).
With the exception of Takechi, who was a Joshi (superior warrior) rank, they all underwent severe torture.
[5] Izō endured severe torture, but he finally made a full confession and was beheaded on May 11, 1865, and his head was put on public display.
The NHK Taiga drama Ryōmaden (2009) featured him several times (portrayed by Takeru Sato) as one of Sakamoto Ryōma's friends and Takechi Hanpeita's assassins.
Masami Kurumada, author of the popular Saint Seiya manga series, more recently added a character to his work, called Capricorn Izō, who is inspired by Okada.
In the game, his full name is revealed to be Okada Izō Nobutoshi and it is explained he uses a Goken style of swordsmanship, meaning to overpower the opponent through physical strength and audacious assaults, much like the Jigen-ryu used by some other hitokiri of the Bakumatsu period.