Typical high-efficiency organic light-emitting diodes (OLEDs) require exotic high-refractive-index (n ≥ 1.8) substrates to enhance the outcoupling of trapped light in the device. One of the exciting possibilities of OLEDs is the use of lightweight flexible plastic substrates, which unfortunately have a low refractive index (n ≤ 1.6). To unlock the full potential of OLEDs on flexible plastic, we report high-efficiency phosphorescent OLEDs using a thin-film outcoupling enhancement method that does not depend on high-index substrates. In these devices, multifunctional anode stacks, consisting of a high-index Ta₂O₅ optical coupling layer, electrically conductive gold layer and hole-injection MoO₃ layer, are collectively optimized to achieve high efficiency. The maximum external quantum efficiency reaches 63% for green, which remains as high as 60% at >10,000 cd m^–2.