Oceanside, California Located on the northern edge of San Diego County, Oceanside is a beachfront community with 3.7 miles of pristine beach that attracts over 4 million visitors annually who surf, swim, jog, bike, tan or stroll the inviting beaches and the beachfront Strand. Oceanside is the third-largest city in San Diego County. The city has a population of 173,303. Together with Vista and Carlsbad, it forms a "Tri-City area." The city is located just south of Camp Pendleton, the busiest military base in the United States. Oceanside 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.