Comprehensive theoretical and experimental studies are reported on organic light-emitting devices (OLEDs) adopting either the conventional high-index indium tin oxide (ITO) electrode or the low-index conducting polymer electrode, either isotropic emitters or emitters having preferentially horizontal emitting dipoles, and different layer structures. Intriguingly, with the use of low-index electrode in the device, in addition to the known suppression of waveguided modes, the surface plasmon modes can also be effectively suppressed with larger emitter-to-metal distances yet with better immunity to accompanied increase of the competing waveguided modes (induced by thicker organic layers) as in the ITO device. As a result, overall coupling efficiencies of OLED internal radiation into substrates can be significantly enhanced over those with ITO electrodes. Through effective extraction of radiation within substrates, green phosphorescent OLEDs adopting both the low-index ITO-free electrode and the preferentially horizontal dipole emitter (with a horizontal dipole ratio of 76%) achieve a high external quantum efficiency (EQE) of up to approximate to 64%. The simulation also predicts that very high EQEs of >= 80% are possible with highly horizontal dipole emitters for all red/green/blue/white OLEDs, clearly revealing the potential of combining low-index transparent electrodes and horizontal dipole emitters for high-efficiency OLEDs.