Bayesian Optimal Phase II Randomized Clinical Trial Design with Delayed Outcomes R package DTEBOP2 version 1.0.3 Implements a Bayesian Optimal Phase II design (DTE-BOP2) for trials with delayed treatment effects, particularly relevant to immunotherapy studies where treatment benefits may emerge after a delay. The method builds upon the BOP2 framework and incorporates uncertainty in the delay timepoint through a truncated gamma prior, informed by expert knowledge or default settings. Supports two-arm trial designs with functionality for sample size determination, interim and final analyses, and comprehensive simulation under various delay and design scenarios. Ensures rigorous type I and II error control while improving trial efficiency and power when the delay effect is present. A manuscript describing the methodology is under development and will be formally referenced upon publication. Software Depends: R (>= 3.5.0) Imports: truncdist, doParallel, foreach, parallel, invgamma Suggests: knitr, rmarkdown, survRM2adapt, ggplot2, reconstructKM, survival, survminer Zhongheng Cai <zhonghengcai123@gmail.com> Comprehensive R Archive Network (CRAN) Zhongheng Cai [aut, cre], Haitao Pan [aut] GPL-3 2025-05-09 application/tgz https://CRAN.R-project.org/package=DTEBOP2 doi:10.32614/CRAN.package.DTEBOP2