UO2 is particularly susceptible to oxidation. The oxidation processes and the phases formed as a result are widely studied. In the oxidised phases defect clusters form, although their structure and properties are unclear. Thus, we examine fluorite based U3O7 phases to identify the types of defect present in a 36 atom unit cell. We predict that at this stoichiometry, defective structures form oxygen clusters and that split quad-interstitial clusters are the most thermodynamically stable defects. These are associated with a volume contraction compared to UO2, in line with experimental measurements. Incorporation of oxygen is always associated with oxidation of uranium atoms to U5+, although a single calculation leads to a U6+ ion as well. As this system is relatively unstable, it is proposed that U4+ and U5+ are the preferred oxidation states at U3O7 stoichiometry.