To investigate the effect of oxygen on the optical and the electrical properties of amorphous InGaZnO (a-IGZO), we prepared thin films by RF magnetron sputtering in various oxygen atmospheres at room temperature and the thin-film transistors (TFTs) were evaluated. The oxygen concentration during the deposition process affected both the optical band-gap and the mobility of a-IGZO-based devices. As the oxygen concentration in the processing chamber during deposition was increased, the optical band-gap and the saturation mobility decreased concurrently. The highest optical band-gap and the best device performance were obtained from the a-IGZO film deposited in an atmosphere of 10 \% oxygen. The a-IGZO film deposited at this condition exhibited an optical band-gap of 3.29 eV and the transistors fabricated with this film revealed a saturation mobility of 2.6 cm$^2$/V$cdot$s, a subthreshold swing of 0.93 V/decade, an on-off current ratio of 10$^7$ and a threshold voltage of 13.9 V.