Antimony selenide (Sb2Se3) is a promising low-cost and low-toxicity photovoltaic material. The electron buffer layer is of great significance for superstrate Sb2Se3 thin film solar cells. Herein, high-performance Sb2Se3 solar cells were fabricated by using sputtered, bandgap-tunable oxygenated cadmium sulfide (CdS:O) as a novel class of electron buffer layers. The optical transparency and energy band levels of CdS:O buffers were precisely adjusted by controlling the oxygen content in CdS:O layers. With the incorporation of an optimized wide band-gap CdS:O electron buffer layer, the device parameters J(SC), V-OC and FF were all improved. An efficiency of as high as 6.29% was achieved. It's found that, besides the reduced light absorption in the CdS:O layer, the high-quality CdS:O/Sb2Se3 junction played a vital role in the efficiency enhancement. The optimization of interfacial band alignment and defects passivation by atomic oxygen both contribute to the improved quality of CdS:O/Sb2Se3 heterojunction. This study demonstrates that the sputtered CdS:O is a promising electron buffer layer for preparation of high-efficiency and large-area Sb2Se3 thin film solar cells.