Oro County was a county of the United States Territory of Kansas that existed from February 7, 1859 to January 29, 1861, when Kansas joined the Union as a state.
In July 1858, gold was discovered along the South Platte River in Arapahoe County of the Territory of Kansas (now in the State of Colorado).
Oro County was named for the Spanish language name for gold, despite the fact that the county lay to the east of the gold mining region.
Following the Republican Party election victories in 1860, the United States Congress admitted Kansas to the Union.
[1] The Kansas Act of Admission excluded the portion of the Kansas Territory west of the 25th meridian west from Washington from the new state, and Oro County and the rest of this region reverted to unorganized territory.