Orange County (officially the County of Orange; often known by its initials O.C.) is a county located in the Los Angeles metropolitan area in Southern California, United States. As of the 2020 census, the population was 3,186,989, making it the third-most-populous county in California, the sixth-most-populous in the United States, and more populous than 19 American states and Washington, D.C. Although largely suburban, it is the second-most-densely-populated county in the state behind San Francisco County. The county's three most-populous cities are Anaheim, Santa Ana, and Irvine, each of which has a population exceeding 300,000. Santa Ana is also the county seat. Six cities in Orange County are on the Pacific coast: Seal Beach, Huntington Beach, Newport Beach, Laguna Beach, Dana Point, and San Clemente.

Orange County is included in the Los Angeles–Long Beach–Anaheim Metropolitan Statistical Area. The county has 34 incorporated cities. Older cities like Tustin, Santa Ana, Anaheim, Orange, and Fullerton have traditional downtowns dating back to the 19th century, while newer commercial development or "edge cities" stretch along the Interstate 5 (Santa Ana) Freeway between Disneyland and Santa Ana and between South Coast Plaza and the Irvine Business Complex, and cluster at Irvine Spectrum. Although single-family homes make up the dominant landscape of most of the county, northern and central Orange County is relatively more urbanized and dense as compared to those areas south of Irvine, which are less dense, though still contiguous and primarily suburban rather than exurban.

The county is a tourist center, with attractions like Disneyland Resort, Knott's Berry Farm, Mission San Juan Capistrano, Huntington Beach Pier, the Richard Nixon Presidential Library and Museum, Modjeska House, Segerstrom Center for the Arts, Yost Theater, Bowers Museum, Balboa Island, Angel Stadium, Downtown Santa Ana, Crystal Cove Historic District, the Honda Center, the Old Orange County Courthouse, the Irvine Ranch Natural Landmarks, and several popular beaches along its more than 40 mi (64 km) of coastline. It is also home to a major research university, the University of California, Irvine (UCI), along with a number of other notable colleges and universities such as Chapman University and Cal State Fullerton.