In 2020, 4,622,029 people traveled through OAK, making it the third-largest airport in the San Francisco Bay Area (after San Francisco and San Jose). The airport is an operating base for Southwest Airlines and a focus city for Allegiant Air. As of June 2019, Southwest has 137 daily departures on peak-travel days of the week making it Southwest’s largest operation in California (by peak daily departures). Alaska Airlines combined with sister-carrier Horizon Air has been the second-busiest carrier at the airport through 2013. In January 2014, Delta overtook Alaska as the airport's No. 2 carrier. It has since been surpassed by Spirit.