O'Neill Dam, Embankment dam in Merced County, United States.

O'Neill Dam is an earthfill and rockfill dam on San Luis Creek, about 19 km west of Los Banos in Merced County, California. Built from 1963 to 1967, it stands 26.7 m high and its crest runs 4,400 m across the valley, forming the O'Neill Forebay below San Luis Dam. A morning-glory spillway with a 195.5 m circumference sits at the reservoir's left bank.

The dam was constructed between 1963 and 1967 by the Bureau of Reclamation as part of a major irrigation project for the valley. Its completion marked an important step in moving water from northern sources to agricultural areas in the south.

The forebay serves as a recreation hub for people from the surrounding region who come for fishing, boating, and weekend outings. It has become a regular gathering place for water lovers and families seeking outdoor activities.

The dam is located roughly 20 kilometers west of Los Banos along a major highway and is easily accessible by car. Visitors should note that the location sits in a dry region and water levels vary seasonally throughout the year.

The dam is managed by a special spillway with a circular funnel design that looks like a giant bathtub drain when in operation. This unusual system regulates water flow in an automatic way without requiring moving parts to function.