Short-term oxygen variability is investigated by analyzing time series measurements from a mooring at the shelf break in the Southern California Bight (SCB). The DO fluctuations observed 10 m above the bottom are well correlated (r2=0.95; p<0.008) with density for most of the measurement period in spring 2005 (101 days) except during two upwelling relaxation events (durations of 14 and 16 days). Spectral peaks at diurnal-inertial and tidal frequencies become weak during the relaxation events. Suboxic conditions, with DO content less than 2.0 ml/l, were observed when density increases as deep slope water moves up toward the shelf under ‘upwelling' conditions. In contrast, during relaxation events, suboxic conditions occurred without such cross-shore isopycnal shoaling due to poleward advection of low-oxygen water along the shelf from the south. These suboxic conditions tend to last longer than corresponding periods during upwelling. We propose a mechanism for creating this different low DO water mass on the broader shelf to the south. Similar to the processes which bring warm water into an upwelling region during relaxations, this mechanism also represents a non-linear rectification of oscillating upwelling/relaxation conditions, resulting in an overall increase in low-oxygen events and durations. The various time scales of natural oxygen variability that we observe in the coastal SCB are predicted to influence aspects of biological diversity, connectivity, productivity and behavior, with significance for management and conservation. Shoreward migrations of mobile animals associated with habitat compression are predicted to occur during exposure to such short-term oxygen depletion as we find.