Ōyama was born in Kiyama, a town located in Japan’s Saga Prefecture, on 9 January 1913. He was the eldest son and the third of nine children born into a farming family. One of his younger brothers died in the East China Sea in 1944, while his youngest brother was still alive as of 2023, being 92 years old.

In 1943, when Ōyama was 30 years old, he was drafted and dispatched to China, leaving his wife and two young daughters behind. Shortly before the war’s end, he went back to Japan, following orders to safeguard the mainland and managing to avoid detainment in Siberia. Upon his return, he was afflicted with a kidney ailment and experienced widespread bodily swelling.

In his 70s, Ōyama battled prostate and heart ailments, but later made a complete recovery. Until the age of 100, he was an active driver of both cars and light trucks, using them for his work in the fields. His family attributed his longevity to his practice of engaging both his mind and body in planning for the upcoming year’s crop after each harvest. Furthermore, his family said he had a calm and positive personality.

Ōyama’s wife passed away in 2018, at the age of 100 years, 11 months. In December 2019, he moved into a care home with rehab facilities. Despite the challenges posed by the COVID-19 pandemic, he managed to maintain regular family visits through acrylic panels.

Ōyama was able to walk with the assistance of a walker until March 2023. In January 2024, he celebrated his 111th birthday at a hospital in his hometown of Kiyama. He died from natural causes on 30 January 2024 at the age of 111 years, 21 days.