The relationship between interfaces, including grain boundaries, and mass transport is complex in the simplest of materials. In complex oxides the situation is even more confounding. In the bulk, mass transport is intricately related to the level of cation disorder. Here, using pyrochlore as a model system, we examine the relationship between grain boundary structure, cation disorder, and mass transport. We find that, for pyrochlores exhibiting ordered cation sublattices, grain boundaries tend to enhance oxygen transport and that enhancement depends on the nature of the boundary. However, as cation disorder is increased, there is a transition from grain boundary to bulk dominated diffusion. In the later regime, as the role of the grain boundaries is completely overwhelmed by the bulk contributions, the character of the grain boundaries is irrelevant. These results have implications for the properties of grain boundaries in complex oxides generally and mass transport in pyrochlore more specifically.