Omega Township is located in Marion County, Illinois. As of the 2010 census, its population was 501 and it contained 212 housing units.
Omega Township (T3N R4E) is centered at 38ð42'N 88ð45'W (38.699, -88.756). Forbes Lake (El. 156 m) and Stephen A. Forbes State Park are located mainly at the north end of the township. According to the 2010 census, the township has a total area of , of which (or 97.69%) is land and (or 2.31%) is water.