During World War II, at the age of 17, Oyama served as a cadet in the Imperial Japanese Army Academy.

In his last year at Waseda University, he started to attend Tokyo Theological Seminary and graduated in 1953.

The KGK is now a nationwide Christian organization in Japan and a member association of IFES (International Fellowship of Evangelical Students).

On March 15, 1953, Oyama stood on a street near Takadanobaba Station in Tokyo and started to preach the gospel to people.

In 1956, Oyama together with others who had a burden for overseas ministries got together from all over the country, and they decided their first overseas missionary work would be to go to these other Asian countries, who were attacked by Japan during World War II, and apologize and make reparations for the destruction and for any atrocities that occurred.

From 1962 to 1966, Oyama shared the gospel on a radio station called Nippon Broadcasting System, Incorporated (ニッポン放送).