Big Oaks National Wildlife Refuge consists of approximately 50,000 acres within Jefferson, Ripley, and Jennings counties in southeastern Indiana. Under a recent agreement with the US Army and US Air Force, the Service operates the refuge through a 25-year real-estate permit. The Army retains ownership of the land (the closed Jefferson Proving Ground). The Air Force retains use of a bombing range which is not included in the portion designated as a national wildlife refuge. Large safety buffer areas separate the Air Force range from public use areas of the refuge.

Big Oaks Refuge contains the largest unfragmented forested block in southeastern Indiana and some of the largest grassland areas found within the region. The refuge provides habitat for 120 species of breeding birds, the Federally endangered Indiana bat, and 41 species of fish. The refuge also is home to white-tailed deer, wild turkey, river otters, and coyotes. Over 25 State-listed animal species and over 46 State-listed plant species have been discovered to date on the refuge. Many bird species of management concern are also found here, including Henslow's sparrows and cerulean warblers. Over 800 singing male Henslow's sparrows use the large grasslands on the refuge. Big Oaks National Wildlife Refuge has been designated as a Globally Important Bird Area because of its value to Henslow's sparrows and other migratory birds.

The entrance to the refuge is located on U.S. Highway 421, five miles north of Madison, Indiana. The refuge office is in Building 125 in the cantonment area of the former Jefferson Proving Ground. From Madison, drive north on Highway 421 to the main entrance of the former Jefferson Proving Ground and go west on Ordnance Drive. Take the first right (Shun Pike) and then a left on Niblo Road. Building 125 is located on 1661 West Niblo Road.