Los Osos Oaks State Natural Reserve, Natural reserve with ancient oaks in Los Osos Valley, California.

Los Osos Oaks State Natural Reserve is a protected area in Los Osos Valley featuring hundreds of coast live oaks growing on sand dunes with intricately twisted branches and gnarled trunks. The vegetation spreads across a dune-filled landscape where trees grow close together and open sandy areas break through the forest canopy.

Los Osos Oaks State Natural Reserve was established as a California State Park in 1972, protecting 85 acres (34 ha) of coast live oaks on relict sand dunes. In 2008 it was among the state parks threatened with closure; after voters rejected budget propositions in the 2009 special election, Governor Arnold Schwarzenegger proposed closing 220 parks for at least two years. The closures were avoided by cutting hours and maintenance system-wide.

The reserve takes its name from the Los Osos Valley, and its oaks grow on old stabilized dunes rather than on ordinary valley soil. Just north of the reserve, the Chumash site of Los Osos Back Bay has been partly excavated on a stabilized sand dune and dates to at least 1200 CE; archaeological material has also been recovered inside the reserve itself.

The reserve lies in Los Osos Valley between San Luis Obispo and Baywood Park-Los Osos, just outside the town of Los Osos, reached from Los Osos Valley Road. Trails wind over stabilized relict sand dunes, so expect sandy, uneven footing rather than flat ground. The adjacent Elfin Forest Natural Area in Baywood-Los Osos extends a visit to the area.

The reserve is the only place on Earth where the critically endangered lichen Sulcaria isidiifera, known as splitting yarn or Iron Maiden's hair, can be found. It grows on the coast live oaks that give the park its name.