The Oxnard Plain is a large coastal plain in southwest Ventura County, California, United States surrounded by the mountains of the Transverse ranges. The cities of Oxnard, Camarillo, Port Hueneme and much of Ventura as well as the unincorporated communities of Hollywood Beach, El Rio, Saticoy, Silver Strand Beach, and Somis lie within the over 200-square-mile alluvial plain (520 km2). The population within the plain comprises a majority of the western half of the Oxnard-Thousand Oaks-Ventura Metro Area and includes the largest city along the Central Coast of California. The 16.5-mile-long coastline (26.6 km) is among the longest stretches of continuous, linear beaches in the state.

Somis is an unincorporated community in Ventura County, California. It was established in 1892 by Thomas Bard and D.T. Perkins on a portion of the Rancho Las Posas Mexican land grant. Somis is in the Las Posas Valley on the south bank of Fox Barranca, just west of Arroyo Las Posas. For statistical purposes, the United States Census Bureau has defined Somis as a census-designated place (CDP).