Los Osos (Spanish for "the bears") is an unincorporated town in San Luis Obispo County, California, United States.

[4] For statistical purposes, the United States Census Bureau has defined Los Osos as a census-designated place (CDP).

There is a large Northern Chumash archaeological site on a stabilized sand dune in Los Osos dating to at least as early as 1200 CE.

[5] The remains of two Northern Chumash people were removed from a site at Sweet Springs Nature Preserve dating to around 1700 BCE, and were later inventoried under NAGPRA and are possibly in the process of repatriation to the individuals' descendants.

[8] On September 7 – 8, 1769, the Portolà expedition traveled through the San Luis Obispo area on their way to rediscover the Bay of Monterey.

[10] The Portolà expedition was the beginning of a push by Spain to explore the northwestern frontier of Las Californias, and to establish both military outposts and missions.

The move was intended to counter what were thought to be colonial designs by Russian traders from Alaska and the British fur companies in the Pacific Northwest.

(See: Fort Ross, Russian-American Company, Hudson's Bay Company) The Portolà expedition was organized by the Visitadór Generál of New Spain, José de Gálvez, following up (belatedly) on previous maritime expeditions, most notably that of Sebastián Vizcaíno in 1602–3, who visited and described many points along the coast, including Monterey Bay, Portolà's primary destination.

The city is located next to the Pacific Ocean, which helps moderate temperatures and create an overall pleasant mild year-round climate, resulting in warmer winters and cooler summers compared with places farther inland, such as Atascadero.

The 90-acre (36 ha) Elfin Forest[14] is on the southeast side of the estuary that lies between Los Osos and Morro Bay State Park.

Large groves of eucalyptus[15] trees attract the annually migrating monarch butterflies to Los Osos.

Though rarely seen in residential neighborhoods, a bear population is centered in the Los Padres National Forest which is some 10 miles (16 km) away.

Since 1983, a section of the community of Los Osos (Prohibition Zone) has been under a septic tank discharge prohibition, Resolution 83–13, issued by the Central Coast Regional Water Quality Control Board because that part of the town's septic tanks are too numerous and concentrated to dissipate nitrates.

San Luis Obispo County was the original authority in charge of building the Wastewater Treatment System.

Due to voiced opposition at the meeting from members of the Los Osos/Baywood Park Community, the Commission postponed its decision until a full hearing could be held.

In 1998 an election was held to form the Los Osos Community Services District (LOCSD) by residents as a response to the high cost of the original sewer proposal.

The Central Coast Regional Water Quality Control Board has used its enforcement powers to impose fines against the district in the amount of $6.6 million for violation of the discharge prohibition emanating from three LOCSD-owned sites.

During February 2006 the Regional Water Quality Control Board, threatened it would begin to issue cease and desist orders to citizens of Los Osos, and may require recipients to pump their septic systems every three years, and to stop using them by 2011.

The California Coastal Commission has denied the county a permit to proceed due to "Substantial Issues" that were cited during an appeals hearing.