Abstract: A series of new bipolar hosts C1O1, C2O1, and C1O2 comprising hole-transporting carbazole as donor (D) and electron-transporting oxadiazole as acceptor (A) have been designed and synthesized to tune D/A ratio with a view for getting in-depth information about structure–property–performance relationship. The D/A ratio exerts subtle influences on absorption spectra and energy levels. Time-of-flight (TOF) measurements reveal that the increment of donor units effectively increase the hole mobility. The appropriate energy levels and triplet energies along with promising morphological and thermal stability of these molecules bring out them as suitable hosts to realize green PhOLEDs with (PBi)2Ir(acac) and (PPy)2Ir(acac) emitters. Green phosphorescent devices hosted by C1O1, C2O1, and C1O2 achieved maximum external quantum efficiencies of 20.7%, 20.4%, and 17.3%, respectively. These results indicate that carbazole/oxadiazole hybrid molecule with one carbazole and oxadiazole each is a better candidate for highly efficient green PhOLEDs.