Ozaki was born in what is now the town of Shirakawa, Gifu Prefecture, and a descendant of a samurai family. His family relocated to Taiwan when he was a youth, and he grew up in Taipei. Growing up in Taiwan left Ozaki with a deep respect and affection for Chinese culture, and was very fond of the island, where he spent his childhood. Ozaki's father worked for the Japanese colonial government and taught his son that as Japan was the most advanced of the Asian nations it had a special "civilizing mission" - not only in Taiwan, but in all of Asia. Ozaki was brought up bilingual, and had an education steeped in the classics of both Japanese and Chinese literature in order to better understand China. Ozaki was opposed to the crude anti-Chinese racism of the Japanese ultra-nationalists, who saw the Chinese as a people fit only to be slaves, which led him to an increasing estrangement from his country as time went on. He returned to Japan in 1922, and enrolled in the Legal department of Tokyo Imperial University. Appalled by the actions of the government in the aftermath of the 1923 Great Kantō earthquake he turned to Marxism. In the aftermath of the Great Kantō earthquake, members of the extreme right-wing groups began vigilante killings of ethnic Koreans and leftists, as supposedly the Koreans and left-wingers were engaging in looting, which the police did nothing to stop. Ozaki was greatly upset at the way that the police tolerated these killings. He left school without graduating in 1925, after becoming involved in the activities of the Japanese Communist Party. In 1926, he joined the "Asahi Shimbun" newspaper, where he wrote articles on Soviet leaders Vladimir Lenin and Joseph Stalin. He was transferred to the Osaka "Mainichi Shimbun" the following year.