Water transport through graphene-derived membranes has gained much interest recently due to its promising potential in filtration and separation applications. In this work, we explore water permeation in graphene oxide membranes using atomistic simulations and theoretical analysis, by considering flow through the interlayer gallery, expanded channels such as wrinkles of interedge spaces, and pores within the sheet. We find that, although flow enhancement can be established by nanoconfinement, fast water transport through pristine graphene channels is prohibited by a prominent side-pinning effect from capillaries formed within oxidized regions. We then discuss several flow enhancement mechanisms through the porous microstructures of graphene oxide membranes. These understandings are integrated into a complete picture to understand water permeation through the layer-by-layer and porous microstructure and can guide rational design of functional membranes for energy and environmental applications.