Ushikubiyama in the southern part of the city, and curves through the city limits until it joins the Mikasa River, which runs beside the Shimo-Ori housing complex.Ducks, turtles, koi, egrets, herons and Japanese wagtails can be seen in or around the river, depending on the season.

Fukuoka Prefecture Ōnojō has a humid subtropical climate (Köppen Cfa) characterized by warm summers and cool winters with light to no snowfall.

The city takes its name from Ōno Castlem which was built in the Asuka Period to protect Daizaifu against possible invasion by Silla and Tang China.

Ōnojō has a mayor-council form of government with a directly elected mayor and a unicameral city council of 20 members.

In terms of national politics, the city is part of the Fukuoka 5th district of the lower house of the Diet of Japan.

Central Ōnojō is on the flight path to Fukuoka airport, so noise pollution can be a problem.