O'Neill Dam and Forebay are joint Federal-State facilities located on San Luis Creek 2.5 miles downstream from San Luis Dam. O'Neill Forebay Dam is a feature of the Central Valley Project - San Joaquin Division - San Luis Unit. The reservoir has a surface area of 2,250 acres and 14 miles of shoreline. The Park includes boating, camping, picnic facilities, fishing, hiking, and serves as a major family destination for many in the west central valley. Currently an expansion of the Maderos boat ramp facilities is underway, which will improve boat access and day use facilities.

The site is located about 10 miles west of Los Banos, California on CA Highway 152.