Driven by innovations in the form factor of organic light-emitting diode (OLED) displays, the application scope of OLED technology now encompasses the biomedical field, in addition to its existing application domains of mobile phones, televisions, and lighting. This paper introduces an ultrathin, ultraflexible, and high-power bio-OLED patch with perfect waterproofing and an elongation of 2.04% through material and structural design. Furthermore, the OLED patch with a parallel-stacked OLED delivers a high output of 100 mW/cm2, achieves a 40% power density improvement compared to glass-based OLEDs using optimized encapsulation, and is suitable for photodynamic therapy owing to its lifetime of 183 h at an intensity of 35 mW/cm2. Since OLED patches are required for long-term stable operation in various biomedical applications, we developed an OLED patch with an outcoupling structure using a simple method. The improved OLED patch achieved a 35% increase in light extraction compared to the original OLED patch.