A series of bipolar transport host materials: 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD) (1), 2,5-bis(4-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (p-CzOXD) (2), 2,5-bis(3-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (m-CzOXD) (3) and 2-(2-(9H-carbazol-9-yl)phenyl)-5-(4-(9H-carbazol-9-yl)phenyl)-1,3,4-oxad iazole (op-CzOXD) (4) are synthesized through simple aromatic nucleophilic substitution reactions. The incorporation of the oxadiazole moiety greatly improves their morphological stability, with T-d and T-g in the range of 428-464 degrees C and 97-133 degrees C, respectively. The ortho and meta positions of the 2,5-diphenyl-1,3,4-oxadiazole linked hybrids (1 and 3) show less intramolecular charge transfer and a higher triplet energy compared to the para-position linked analogue (2). The four compounds exhibit similar LUMO levels (2.55-2.59 eV) to other oxadiazole derivatives, whereas the HOMO levels vary in a range from 5.55 eV to 5.69 eV, depending on the linkage modes. DFT-calculation results indicate that 1, 3, and 4 have almost complete separation of their HOMO and LUMO levels at the hole- and electron-transporting moieties, while 2 exhibits only partial separation of the HOMO and LUMO levels possibly due to intramolecular charge transfer. Phosphorescent organic light-emitting devices fabricated using 1-4 as hosts and a green emitter, Ir(ppy)(3) or (ppy)(2)Ir(acac), as the guest exhibit good to excellent performance. Devices hosted by o-CzOXD (1) achieve maximum current efficiencies (eta(c)) as high as 77.9 cd A(-1) for Ir(ppy)(3) and 64.2 cd A(-1) for (ppy)(2)Ir(acac). The excellent device performance may be attributed to the well-matched energy levels between the host and hole-transport layers, the high triplet energy of the host and the complete spatial separation of HOMO and LUMO energy levels.