Post-OPEX Cross-Sectional Effect: Final Historical Audit

A frozen-candidate falsification study of US equity returns in the first two trading sessions after monthly options expiration, using causal timing, cost stress, placebo controls, cycle-level bootstrap, sub-universe replication and immutable forward governance. The study evaluates the historical robustness of a frozen Post-OPEX cross-sectional strategy while explicitly preserving the original research gate and separating historical evidence from true forward out-of-sample validation.