In this paper, amorphous indium-gallium-oxide solar-blind metal-semiconductor-metal photodetectors were fabricated by using co-sputtering method. Three samples with different oxygen concentrations, namely, sample A without oxygen, sample B with 2% oxygen concentration, and sample C with 4% oxygen concentration, were investigated. The applied bias was 5 V during device characterization. The ultraviolet (UV)-to-visible rejection ratios were 39, 9.9 × 103, and 1.1 × 105 for samples A, B, and C, respectively. The dynamic responses of the decay times were 5.79/52.12 s, 1.96/30.49 s, and 0.02/0.75 s for samples A, B, and C, respectively. In summary, the 4% oxygen concentration sample exhibited excellent UV-to-visible rejection ratio and superior decay time.