Insufficient mechanical properties are one of the major obstacles for the commercialization of ultrahigh permeability thermally rearranged (TR) membranes in large-scale gas separation applications. The incorporation of preformed benzoxazole/benzimidazole units into o-hydroxy copolyimide precursors, which themselves subsequently thermally rearrange to form additional benzoxazole units, were prepared for the first time. Using commercially available monomers, mechanically tough membranes prepared from random and block TR poly(benzoxazole-co-imide) copolymers (TR-PBOI) were investigated for gas separation. The effects of the chemical structures, copolymerization modes, and thermal holding time of o-hydroxy copolyimides on the molecular packing and properties, including gas transport, for the resulting TR-PBOI membranes have been examined in detail. After treatment at 400 °C, tough TR-PBOI membranes exhibited tensile strengths of 71.4−113.9 MPa and elongation at break of 5.1−16.1%. Moreover, they presented higher or comparable gas transport performance as compared to those tough/robust TR membranes reported previously. Reported for the first time is a comparative investigation of the copolymerization mode (random or block) on membrane properties. The novel polymer architecture and systematic property studies promote a better understanding of the materials and process development of commercial TR membranes for gas separation applications.