Oneco Township is located in Stephenson County, Illinois. As of the 2010 census, its population was 1,331 and it contained 587 housing units. The village of Orangeville, and the unincorporated community of Oneco are located in this township.
Oneco is Township 29 North, Range 7 and 8 (part) East of the Fourth Principal Meridian.
According to the 2010 census, the township has a total area of , all land.