O'Neill Dam is an earthfill dam on San Luis Creek, 12 miles (19 km) west of Los Banos, California, United States, on the eastern slopes of the Pacific Coast Ranges of Merced County.

Built from 1963 to 1967, the dam is an earthfill and rockfill construction stretching over 3 miles (4.8 km) across the valley of San Luis Creek.

The O'Neill Forebay reservoir is fed by releases from the San Luis Dam as well as from the Delta–Mendota Canal.

[1] The peak inflow to the forebay is 15,600 cubic feet (440 m3) per second, from both the San Luis Dam and the Delta–Mendota Canal.

Drainage area of the reservoir downstream of the San Luis Dam is only 18 acres (7.3 ha).