Oso Creek is an approximately 13.5-mile (21.7 km) tributary of Arroyo Trabuco in southern Orange County in the U.S. state of California. Draining about 20 square miles (52 km2) in a region north of the San Joaquin Hills and south of the Santa Ana Mountains, the creek is Trabuco Creek's largest tributary, and is part of the San Juan Creek drainage basin. Beginning in the foothills of the Santa Ana Mountains near the city of Mission Viejo, the creek is dammed twice to form Upper Oso Reservoir and Lake Mission Viejo. The creek is channelized and polluted along much of its length.
"Oso", meaning bear in the Spanish language, was likely the name given to the creek by Spanish conquistadors. Up to the 1970s, the Oso Creek watershed was mostly undeveloped and the creek ephemeral. The watershed lies close to two major wilderness areas - Aliso and Wood Canyons Wilderness Park to the southwest and O'Neill Regional Park to the west, on Trabuco Creek - but has no major parks within its boundaries. Interstate 5 parallels the creek for over half of its length.