[1] The total area of the city is 126.73 km2 (48.93 sq mi) Ōmura is located in former Hizen Province.

It was the site of considerable foreign trade and missionary activity during the late Muromachi period, and the Catholic saint Marina de Omura hails from this city.

Due to its proximity to the trading settlement at Dejima in Nagasaki, was one of the first areas of Japan to re-open to foreign contact after the end of the national seclusion policy after the Meiji restoration.

The former naval base was the location of the squalid[2] Ōmura Migrant Detention Center, where mainly Korean refugees—termed "stowaways" (mikkōsha synonymous with "smuggler")—were held until deportation, frequently for several years.

Since August 1996 the nearby Ōmura Immigration Reception Center in a modern building serves the same function.

Ōmura has a mayor-council form of government with a directly elected mayor and a unicameral city council of 25 members.

In terms of national politics, the city is part of the Nagasaki 3rd district of the lower house of the Diet of Japan.

However, due to its proximity to the larger urban centers of Nagasaki and Sasebo, Ōmura also serves as bedroom community for both cities.