Previous work has shown that the Oregon shelf is a sink for atmospheric carbon dioxide (CO2) during the upwelling season; however, until now, summertime variability in CO2 exchange and sign of the flux for the rest of the year were unknown. Observations of the partial pressure of CO2 (pCO2) in surface waters from August 2007 to May 2010 from ships and a buoy were used with historical data to produce a composite seasonal cycle for the central Oregon midshelf. These data indicate that the region is highly variable, at times being either a sink or strong source for atmospheric CO2. Interannual wind variability was an important determining factor in shaping the sink/source nature of this system. Late summer and early autumn was most variable relative to the rest of the year. Winter pCO2 was near or slightly below atmospheric levels. Strong shelf-wide undersaturated conditions were first observed in early spring and lasted until the upwelling season became developed. Peak upwelling season pCO2 ranged from <200 μatm to >1000 μatm. In July 2008, ship and buoy data revealed previously unobserved high-pCO2 waters (∼1000 μatm) at the surface. These conditions persisted for nearly 2 months and drove this system to be only a weak net annual atmospheric CO2 sink of −0.3 ± 6.8 mol m−2 yr−1. These data showed, for the first time, the seasonal cycle of surface ocean pCO2 on the central Oregon midshelf and the impact of heretofore undocumented pCO2 levels on an estimate of sea-air CO2 flux for this region.