This study addresses the occurrence, severity, and extent of hypoxia over the continental shelf of the northern California Current (40–48.5°N latitude) from 1998 to 2012. Clear seasonal trends exist in the timing and duration of hypoxia. The highest bottom-water dissolved oxygen concentrations occurred from November to March, and levels below the 1.4 mL L−1 hypoxia threshold were detected during the upwelling season (May through October). Regions of hypoxia tended to occur north of 42°N latitude and were most severe over the widest areas of the continental shelf. Hypoxic waters covered up to 62% (15,600 km2) of the continental shelf during some years (2002, 2007, 2008), and < 10% in others (2003, 2010–2011). There are seasonal and interannual shifts in both the depth and oxygen content of upwelled waters. Years with the greatest extent and severity of hypoxia had the lowest concentration of dissolved oxygen in upwelling source waters. The dissolved oxygen concentration of source waters showed a gradual decline from 1998 through 2007, with a subsequent increase from 2008–2010 before leveling off in 2011 and 2012. The variability in oxygen content of source waters correlates well with the North Pacific Gyre Oscillation and may indicate the importance of large-scale climate dynamics in determining the extent and severity of hypoxia in the northern California Current.