Commercial pulse oximetry devices are limited by rigid structures, high manufacturing costs, and reduced sensitivity owing to changes in various conditions. Herein, we propose a method to extend the near-infrared (NIR) detection performance of organic photodetectors (OPDs) by introducing a resonant cavity-enhanced (RCE) structure without synthesizing novel NIR-absorbing organic semiconductor materials. By controlling the thickness of the active layer using PTB7-Th and IEICO-4F, the RCE effect was induced, extending the responsivity and shot-noise-based specific detectivity to approximately 970 nm. Additionally, the RCE-OPD fabricated on an ultraflexible parylene substrate maintained the performance under mechanical deformation and enabled stable pulse oximetry signal under low light intensity conditions. We propose an effective strategy for realizing high-performance flexible OPDs, enhancing the potential of commercializing organic pulse oximeter healthcare monitoring devices.