Eastern Boundary Upwelling Systems like the California Current System are among the ocean’s most productive regions, shaped by distinct surface and subsurface currents and water masses. They are also vulnerable to deoxygenation, with several systems experiencing extreme low-oxygen events and associated mass mortality in recent decades. Determining potential drivers is complicated by seasonal dynamics, multiple water sources, and biological consumption. In the northern California Current System, oxygenated subarctic water mixes with oxygen depleted water from the south. We analyse three 20-year records, including one synthesized from local data, to provide valuable insights into subsurface oxygen variability. These long coastal time series are rare. We show that on the slope in spring, subsurface oxygen variations are physically driven by the relative contributions of these water masses, and their salinity and temperature ratios (a property called `spiciness') can be used to estimate oxygen levels. Seasonally, decreasing spiciness provides evidence of springtime oxygen renewal on the shelf, before respiration consumes oxygen over summer. Since 2003, mid-depth isopycnals show increased southern water contributions in spring and summer, leading to lower oxygen in spring, but not summer, suggesting summer-time oxygen consumption may have decreased with time. On the densest isopycnal near the shelf floor (~ 200 m), where oxygen is already lowest, no changes in spiciness or oxygen have been observed. Future research should examine the use of temperature and salinity ratios to estimate oxygen levels in the wider California Current System, and how changing source water conditions and biological activity may affect the risk of low oxygen in coastal waters.