Old Ocean is an unincorporated community in Brazoria County, Texas, United States.
Old Ocean was originally known as Chance's Prairie, after the land grant made to Joseph H. Polley and Samuel Chance on July 27, 1824. William B. Sweeny, one of the founders of Chance's Prairie and for which the local town of Sweeny was named, bought 2,319 acres (9.38 km2) from Polley and Chance for $1,950. Eventually more people settled here, and the town reportedly had a population of 18 in 1880. James Abercrombie discovered oil nearby in 1934, and a government oil refinery was constructed to make high-octane gas. It was closed near the end of World War II. The oilfield was called Old Ocean, and by 1936 the town eventually took that name. A post office with the name of Old Ocean was established in 1945.