In this paper, we investigate the carrier injection and transport characteristics in iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) doped phosphorescent organic light-emitting devices (OLEDs) with oxadiazole (OXD) as the bipolar host material of the emitting layer (EML). When doping Firpic inside the OXD, the driving voltage of OLEDs greatly decreases because FIrpic dopants facilitate electron injection and electron transport from the electron-transporting layer (ETL) into the EML. With increasing dopant concentration, the recombination zone shifts toward the anode side, analyzed with electroluminescence (EL) spectra. Besides, EL redshifts were also observed with increasing driving voltage, which means the electron mobility is more sensitive to the electric field than the hole mobility. To further investigate carrier injection and transport characteristics, FIrpic was intentionally undoped at different positions inside the EML. When FIrpic was undoped close to the ETL, driving voltage increased significantly which proves the dopant-assisted-electron-injection characteristic in this OLED. When the undoped layer is near the electron blocking layer, the driving voltage is only slightly increased, but the current efficiency is greatly reduced because the main recombination zone was undoped. However, non-negligible FIrpic emission is still observed which means the recombination zone penetrates inside the EML due to certain hole-transporting characteristics of the OXD.