This study presents a novel approach to create waterproof and ultraflexible organic photovoltaics (OPVs) by in-situ growing a hole-transporting layer (HTL) of AgOx. The method enhances the adhesion between the active layer and anode, leading to improved waterproofness without compromising mechanical flexibility and conformability. The in-situ grown AgOx HTL is deposited directly on the active layer and then annealed at 85°C for 24 hours, resulting in a 3 μm-thick OPV that retains 89% and 96% of its initial performance after 4 hours of water immersion and 300 stretching/releasing cycles at 30% strain underwater, respectively. The devices also withstand machine-washing tests, demonstrating their robustness. The strategy is universal, applicable to various active layers, and significantly improves the waterproofness of OPVs by strengthening the interface adhesion. The OPVs exhibit a champion efficiency of 14.3% under one sun illumination, outperforming existing waterproof OPVs. The study highlights the potential of this approach for wearable electronics and underwater applications, offering a promising solution for waterproof and flexible solar cells.This study presents a novel approach to create waterproof and ultraflexible organic photovoltaics (OPVs) by in-situ growing a hole-transporting layer (HTL) of AgOx. The method enhances the adhesion between the active layer and anode, leading to improved waterproofness without compromising mechanical flexibility and conformability. The in-situ grown AgOx HTL is deposited directly on the active layer and then annealed at 85°C for 24 hours, resulting in a 3 μm-thick OPV that retains 89% and 96% of its initial performance after 4 hours of water immersion and 300 stretching/releasing cycles at 30% strain underwater, respectively. The devices also withstand machine-washing tests, demonstrating their robustness. The strategy is universal, applicable to various active layers, and significantly improves the waterproofness of OPVs by strengthening the interface adhesion. The OPVs exhibit a champion efficiency of 14.3% under one sun illumination, outperforming existing waterproof OPVs. The study highlights the potential of this approach for wearable electronics and underwater applications, offering a promising solution for waterproof and flexible solar cells.