In this study, we diagnose the interannual-to-decadal variability of ocean CO2 uptake from three independent methods: an ocean circulation inverse model (OCIM), global ocean biogeochemical models (GOBMs), and pCO2-based flux mapping products. We find that the ocean carbon sink could be responsible for up to 40% of the observed decadal variability in atmospheric CO2 accumulation. Data-based estimates of the ocean carbon sink from pCO2 mapping methods and decadal ocean inverse models generally agree on the magnitude and sign of decadal variability in the ocean CO2 sink at both global and regional scales. Simulations with ocean biogeochemical models confirm that climate variability drove the observed decadal trends in ocean CO2 uptake, but also demonstrate that the sensitivity of ocean CO2 uptake to climate variability may be too weak in models. Finally, we discuss the relative contribution of atmospheric pCO2, solubility, circulation, and biology to the decadal variability of the ocean CO2 sink.