Old Bethpage is a hamlet and census-designated place located on Long Island in the Town of Oyster Bay, Nassau County, New York, USA. The population of the CDP was 5,523 at the 2010 census. It is served by the Old Bethpage Post Office, ZIP code 11804.Old Bethpage and its neighboring hamlet, Plainview, share a school system, library, fire department and water district. Law enforcement for the community is provided by the Nassau County Police Department's Eighth Precinct.