O'Fallon Township is located in St. Clair County, Illinois.

As of the 2010 census, its population was 26,053 and it contained 10,338 housing units.

[2] According to the 2010 census, the township has a total area of 35.78 square miles (92.7 km2), of which 35.12 square miles (91.0 km2) (or 98.16%) is land and 0.66 square miles (1.7 km2) (or 1.84%) is water.

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