© 2019 IEEE. We demonstrate a Si-face 4H-SiC MOSFET with oxygen (O) doping in the channel region for the first time. Compared with a conventional device, the O-doped channel was found to provide lower channel resistance (Rch) and higher threshold voltage (Vth), which is expected from the fact that O acts as a deep level donor in 4H-SiC. By applying this novel technique to vertical 4H-SiC MOSFETs, 32 % reduction of specific on resistance (Ron) at a high Vth of 4.5 V was achieved. In order to evaluate gate oxide reliability, negative bias temperature instability (NBTI) of Vth was investigated. The O-doped channel shows a smaller Vth shift, and its acceleration coefficient of the time to Vth shift is similar to that of a conventional one. Therefore, the O-doped channel is found to be a promising approach to further improve NBTI of 4H-SiC MOSFETs by channel engineering using deep level donors.