The response of the Madden-Julian oscillation (MJO) to ocean feedbacks is studied with coupled and uncoupled simulations of four general circulation models (GCMs). Monthly mean sea surface temperature (SST) from each coupled model is prescribed to its respective uncoupled simulation, to ensure identical SST mean-state and low-frequency variability between simulation pairs. Consistent with previous studies, coupling improves each model's ability to propagate MJO convection beyond the Maritime Continent. Analysis of the MJO moist static energy budget reveals that improved MJO eastward propagation in all four coupled models arises from enhanced meridional advection of column water vapor (CWV). Despite the identical mean-state SST in each coupled and uncoupled simulation pair, coupling increases mean-state CWV near the equator, sharpening equatorward moisture gradients and enhancing meridional moisture advection and MJO propagation. CWV composites during MJO and non-MJO periods demonstrate that the MJO itself does not cause enhanced moisture gradients. Instead, analysis of low-level subgrid-scale moistening conditioned by rainfall rate (R) and SST anomaly reveals that coupling enhances low-level convective moistening for R> 5 mm day(-1); this enhancement is most prominent near the equator. The low-level moistening process varies among the four models, which we interpret in terms of their ocean model configurations, cumulus parameterizations, and sensitivities of convection to column relative humidity.