High Quality Content by WIKIPEDIA articles! Oceanside is a coastal resort city located on California`s South Coast. It is the third-largest city in San Diego County, California. The city had a population of 183,095 at the 2010 census. Together with Carlsbad and Vista, it forms a tri-city area. Oceanside is located just south of Marine Corps Base Camp Pendleton, the busiest military base in the United States. The city has experienced dramatic growth since 1970, when its population was 45,000. Much of the city`s area was developed into single-family home tracts during the 1970s and 1980s. Since the 1990s, increased commercial and industrial development have diversified Oceanside`s economic base.