High Quality Content by WIKIPEDIA articles! Oso Creek is an approximately 13.5-mile (21.7 km) tributary of Trabuco Creek 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.