O'Brien is a city in Haskell County, Texas, United States.

O'Brien is located at the junction of Farm to Market Road 2229 and Texas State Highway 6; the nearest large city is Abilene, 70 miles to the south.

According to the United States Census Bureau, the city has a total area of 0.5 square miles (1.3 km2), all land.

The City of O'Brien is served by the Knox City-O'Brien Consolidated Independent School District.

In 1972, O'Brien won the first six-man football state championship in Texas.