The Double-O Ranch Historic District is located east of Harney Lake in Harney County in southeastern Oregon, United States. At one time, the Double-O Ranch covered over 17,000 acres. The ranch was owned by Bill Hanley, a well-known cattle baron and Bull Moose progressive. In 1941, the United States Government purchased most of the Double O Ranch property and added it to the Malheur National Wildlife Refuge. The two remaining Double-O Ranch buildings are now listed on the National Register of Historic Places.