Randomized clinical trials are the gold standard to evaluate the efficacy of an experimental treatment. We propose a flexible Bayesian optimal phase II (BOP2) design for two-arm randomized trials. The proposed two-arm BOP2 design is flexible and can handle single, multiple primary and coprimary endpoints for superiority and noninferiority trials under a unified framework. It also allows users to specify the number and timing of interim analyses to meet clinical needs. While enjoying the flexibility...