According to the 2021 census gazetteer files, Onarga has a total area of 1.67 square miles (4.33 km2), all land.

[4] Onarga has a hot-summer humid continental climate (Dfa) characterized by four distinct seasons.

[5] Spring is generally mild but often marked by rain, wind, and cloudy skies, with high temperatures increasing from the 50s in late March to the 80s by early June.

Onarga's summer conditions are particularly favorable for cultivating corn and soybeans, which dominate the surrounding rural landscape.

Because of its close proximity to Spring Creek, Onarga was one of the first settled areas of Iroquois County.

"[citation needed] Many fields surrounding the town bear rows of trees and bushes rather than corn and soybeans, which are otherwise ubiquitous throughout the region.

The prosperous nursery businesses have led to a great amount of ethnic diversity in Onarga, as many Mexican and Mexican-American migrant workers have chosen to maintain roots in the community.

Onarga celebrated its sesquicentennial with a nearly week-long festival in the summer of 2004, which included social events and a strong focus on the history of the community.

Onarga is the final resting place of Civil War spy and Pinkerton detective Timothy Webster.