This study presents a novel approach to enhance the waterproofness and mechanical flexibility of ultraflexible organic photovoltaics (OPVs) by in-situ growing a hole-transporting layer (HTL) of AgOx. The in-situ growth process involves directly depositing Ag onto the active layer followed by thermal annealing, which strengthens the interface adhesion between the active layer and the anode. This method significantly improves the waterproof performance of the OPVs, maintaining 89% and 96% of their initial efficiency after 4 hours of immersion and 300 stretching/releasing cycles at 30% strain underwater, respectively. The devices also withstand machine-washing tests, demonstrating their suitability for wearable and underwater applications. The strategy is universal, showing improved waterproofness in various active layer materials, and enhances the overall stability and reliability of the OPVs.This study presents a novel approach to enhance the waterproofness and mechanical flexibility of ultraflexible organic photovoltaics (OPVs) by in-situ growing a hole-transporting layer (HTL) of AgOx. The in-situ growth process involves directly depositing Ag onto the active layer followed by thermal annealing, which strengthens the interface adhesion between the active layer and the anode. This method significantly improves the waterproof performance of the OPVs, maintaining 89% and 96% of their initial efficiency after 4 hours of immersion and 300 stretching/releasing cycles at 30% strain underwater, respectively. The devices also withstand machine-washing tests, demonstrating their suitability for wearable and underwater applications. The strategy is universal, showing improved waterproofness in various active layer materials, and enhances the overall stability and reliability of the OPVs.