Oensingen is a municipality in the district of Gäu in the canton of Solothurn in Switzerland.

[3] Oensingen has an area, as of 2009[update], of 12.07 square kilometers (4.66 sq mi).

[4] The municipality is located in the Gäu district, at the base of the Jura Mountains at the exit of the Balsthal gorge.

The blazon of the municipal coat of arms is Gules a Cross pattee couped Argent.

[8] Most of the population (as of 2000[update]) speaks German (3,859 or 85.4%), with Serbo-Croatian being second most common (164 or 3.6%) and Albanian being third (153 or 3.4%).

[7] The age distribution, as of 2000[update], in Oensingen is; 354 children or 7.8% of the population are between 0 and 6 years old and 739 teenagers or 16.4% are between 7 and 19.

[8] The historical population is given in the following chart:[3][13] The prehistoric and Roman era hilltop settlement and medieval castle at Lehnflue and Neu Bechburg Castle are listed as Swiss heritage sites of national significance.

In the federal election, a total of 1,377 votes were cast, and the voter turnout was 46.8%.

[8] There were 2,509 residents of the municipality who were employed in some capacity, of which females made up 42.7% of the workforce.

The number of jobs in the primary sector was 37, of which 36 were in agriculture and 1 was in forestry or lumber production.

In the tertiary sector; 936 or 36.7% were in wholesale or retail sales or the repair of motor vehicles, 321 or 12.6% were in the movement and storage of goods, 86 or 3.4% were in a hotel or restaurant, 31 or 1.2% were in the information industry, 45 or 1.8% were the insurance or financial industry, 165 or 6.5% were technical professionals or scientists, 54 or 2.1% were in education and 594 or 23.3% were in health care.

[8] From the 2000 census[update], 1,893 or 41.9% were Roman Catholic, while 1,242 or 27.5% belonged to the Swiss Reformed Church.

The education system in the Canton of Solothurn allows young children to attend two years of non-obligatory Kindergarten.

All the lower secondary students from Oensingen attend their school in a neighboring municipality.

The library has (as of 2008[update]) 15,633 books or other media, and loaned out 53,815 items in the same year.

It was open a total of 235 days with average of 15 hours per week during that year.