O'Neill Forebay is a forebay to the San Luis Reservoir created by the construction of O'Neill Dam across San Luis Creek approximately 12 miles (19 km) west of Los Banos, California, United States, on the eastern slopes of the Pacific Coast Ranges of Merced County.

Roughly 2.5 miles (4.0 km) downstream from the San Luis Dam, O'Neill Forebay collects irregular water releases from the San Luis Dam and the 424 MW pumped-storage hydroelectric William R. Gianelli Powerplant in its 56,400 acre⋅ft (69,600 dam3) basin.

The drainage area of the reservoir downstream of the San Luis Dam is only 18 acres (730 dam2).

[1] The California Office of Environmental Health Hazard Assessment (OEHHA) has developed an advisory for O'Neill Forebay because of mercury and PCBs found in fish caught here.

The fish, 52.5 in (1,330 mm) and weighing 70.6 lb (32.0 kg), was caught on August 5, 2008, by Frank Ualat of Gilroy, California.