We investigated the relationship between the structure and misorientation angle of (001) twist grain boundaries in Bi2Sr2Ca1Cu2Oy/Ag composite tapes processed in different oxygen partial pressures (PO2 = 0.01, 0.21, and 1 atm). Large-angle misoriented twist boundaries (>10°) essentially had no amorphous layers at the interface, and the misorientation angles of these boundaries mostly corresponded to low-energy misorientations. This large-angle misoriented boundary structure was independent of PO2. Small-angle misoriented twist boundaries (<10°), on the other hand, corresponded to high-energy misorientations and sometimes had amorphous layers at the interface. The population of the small-angle boundary with an amorphous layer was very low in the tape processed in PO2 = 1 atm. This suggests that high PO2 during the heat treatment is effective in the improvement of grain coupling, and hence, to increase critical current density.