Endohedral clusterfullerenes (ECFs) are fullerene cages with various metallic clusters trapped inside. So far, the actinide-based ECFs are rather scarce with their possible structures and chemistry remaining largely unexplored. Herein, density functional theory calculations characterized that the recently synthesized U2O@C-76 could be U2O@C-s(17 490)-C-76 or U2O@C-2v(19 138)-C-76, whose cages have two or one pentagon adjacencies (PAs) and thus both violate the isolated pentagon rule (IPR). It is noteworthy that they are the first actinide-based ECFs bearing non-IPR outer cages. They are also the first C-s(17 490)- and C-2v (19 138)-C-76-based oxide ECFs. Moreover, U2O@C-2v(19 138)-C-76 is the first example of a hexavalent metal cluster within the C-2v(19 138)-C-76 cage. Interestingly, although trapped by the two same-sized cages, the U2O unit exhibits a bent and a perfect linear configuration, respectively, indicative of the crucial role of cage shape in steering the internal cluster configuration. Their electronic structures can be formally described as (U2O)(6+)@C-76(6-) with primary electrostatic attractions and secondary covalent interactions between cluster and cage. Significantly, bonding analyses reveal that the encaged U2O moiety may only features two three-center, two-electron (3c-2e) U-O-U bonds with completely absent common two-center bonds.