Onarga is located at 40°42?56?N 88°0?25?W / 40.71556°N 88.00694°W / 40.71556; -88.00694 (40.715432, -88.006928). According to the United States Census Bureau, the village has a total area of 1.3 square miles (3.4 km²), all of it land.
Onarga has a continental climate, with four distinct seasons. Onarga experiences cold winters, with frequent snowfall and temperatures that sometimes plunge to -20°F or colder. Average January high temperatures are generally in the 20s. Record amounts of snowfall for the area were measured in the Winter of 2007-08. Spring is mild, and often rainy, windy, and cloudy with high temperatures ranging from the 50s to 80s. Summers are hot and humid, generally with brief periods of intense thunderstorm and rainfall. Average summer highs are usually in the mid 80s. Falls are crisp and drier, and first killing frosts generally come in October. Onarga's typical summer weather is highly conducive to growing corn and soybeans, which dominate the rural landscape. On average, 40 inches of precipitation per year fall at Onarga.