| Name Official: | O'Neill Dam |
| Dam Crosses: | San Luis Creek |
| Res Name: | O'Neill Forebay |
| Location: | Merced County, California, U.S. |
| Operator: | US Bureau of Reclamation |
| Dam Length: | 14300feet |
| Dam Height: | 87.5feet |
| Dam Width Base: | 124feet |
| Res Capacity Total: | 56400acre.ft |
| Res Catchment: | 18mi2 |
| Res Surface: | 2250acres |
| Coordinates: | 37.0805°N -121.0472°W |
O'Neill Dam is an earthfill dam on San Luis Creek, west of Los Banos, California, United States, on the eastern slopes of the Pacific Coast Ranges of Merced County. Forming the O'Neill Forebay, a forebay to the San Luis Reservoir, it is roughly downstream from the San Luis Dam.
Built from 1963 to 1967, the dam is an earthfill and rockfill construction stretching over across the valley of San Luis Creek. A morning-glory type spillway lies at the left bank of the reservoir. At high, with a maximum reservoir depth of, the crest of the dam is long, at an elevation of . The spillway is, as mentioned before, a morning-glory (inverted bell) design, capacity per second, and with a circumference of .
See main article: O'Neill Forebay.
The O'Neill Forebay reservoir is fed by releases from the San Luis Dam as well as from the Delta–Mendota Canal. Water from the Delta–Mendota Canal is lifted a vertical distance of into a channel running into the forebay.[1] The peak inflow to the forebay is 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 .
The O'Neill Pumping-Generating Plant produces .[1] Irregular water releases from the San Luis Dam and William R. Gianelli Powerplant are collected in the reservoir of the O'Neill Dam, which has a capacity of 56400acre.ft.