Microporous polymers, such as polymers of intrinsic microporosity (PIMs) and thermally rearranged (TR) polymers, have shown promise in advancing the performance of polymer gas separation membranes to overcome the permeability–selectivity trade-off. In this work, a series of thermally rearranged pentiptycene-based polybenzoxazole (PPBO) polymers were prepared from a new pentiptycene-based poly(o-hydroxyl imide) (PPHI) precursor using carefully designed thermal protocols. Fundamental structure–property relationships within the series were established by comprehensively examining the effects of intermediate treatment temperature and the heating rate on the membrane microporosity, properties, and gas separation performance. The incorporation of bulky pentiptycene units into TR PPBO structures, along with optimized TR thermal protocols in this study, provided a route to finely tune and eventually maximize the separation performance of PPBOs, with several films far exceeding the 2015 upper bound for H₂/CH₄ and O₂/N₂. In CO₂/CH₄ mixed-gas permeation tests, PPBO membranes showed excellent resistance to plasticization under CO₂ partial pressure as high as 6.6 atm, far surpassing the 2008 mixed-gas upper bound for CO₂/CH₄. Moreover, a 5 month aged PPBO film maintained its superior separation performance above the 2008 O₂/N₂ upper bound and 2015 H₂/CH₄ upper bound, indicating the excellent aging resistance of PPBOs.