Different bulk structures of graphite oxide were systematically investigated using density functional theory (DFT). Our model consisted of a hexagonal in-plane structure of graphene with hydroxyl and epoxide groups, and different oxidation levels and water content. The graphitic AB stacking order was stable in anhydrous graphite oxide, independent of oxidation levels. The hydrogen bonding interaction of layers became weaker as the oxidation level increased to the saturation limit. When water molecules were present in highly oxidized graphite oxide, the AB stacking order was broken due to entropic disorder. The interlayer distances increased with the oxidation level: the interlayer distance was 5.1 A for low oxidation graphite oxide and 5.8 A for high oxidation graphite oxide. The calculated interlayer distance of hydrated graphite oxide was 7.3 A, which is in excellent agreement with experimental observations.