In 2006, Oyama became the second most populous city in Tochigi Prefecture, with the capital Utsunomiya retaining the number one spot.

On April 18, 1963, Oyama annexed the town of Mamada and village of Mita, both from Shimotsuga District.

Oyama has a mayor-council form of government with a directly elected mayor and a unicameral city legislature of 40 members.

Oyama, together with the town of Nogi collectively contributes five members to the Tochigi Prefectural Assembly.

In terms of national politics, the town is part of Tochigi 4th district of the lower house of the Diet of Japan.

[5] One of the major employers in the city is Komatsu making iron castings, diesel engines, fork lift trucks and other hydraulic equipment.