Al2O3 was deposited on n-type 4H-SiC by reactive-ion-sputtering (RIS) at room temperature using aluminum target and oxygen as a reactant gas. Post deposition oxygen annealing was carried out at a temperature of 1100 °C. Metal-oxide-semiconductor (MOS) test structures were fabricated on 4H-SiC using RIS-Al2O3 as gate dielectric. The C-V characteristics reveal a significant reduction in flat band voltage for oxygen annealed RIS-Al2O3 samples (Vfb = 1.95 V) compared to as-deposited Al2O3 samples (Vfb > 10 V), suggesting a reduction in negative oxide charge after oxygen annealing. Oxygen annealed RIS-Al2O3 samples also showed significant improvement in I-V characteristics compared to as-deposited RIS-Al2O3 samples. A systematic analysis was carried out to investigate the leakage current mechanisms present in oxygen annealed RIS-Al2O3 on 4H-SiC at higher gate electric field and at different operating temperature. For measurement temperature (T) < 303 K, Fowler-Nordheim (FN) tunneling was found to be the dominant leakage mechanism and for higher temperature (T ≥ 303 K), a combination of FN tunneling and Poole-Frenkel (PF) emission was confirmed. The improvement in I-V characteristics of oxygen annealed RIS-Al2O3/4H-SiC MOS devices is attributed to large effective barrier height (ΦB = 2.53 eV) at Al2O3/SiC interface, due to the formation of an interfacial SiO2 layer during oxygen annealing, as confirmed from X-ray Photoelectron Spectroscopy results. Further improvement in C-V characteristics for oxygen annealed RIS-Al2O3/4H-SiC MOS devices was observed after forming gas annealing at 400 °C. © 2016 Elsevier B.V. All rights reserved.