We give a brief introduction to the oxide (ZnO, TiO2 , In2O3 , SnO2 , etc.)-based magnetic semiconductors from fundamental material aspects through fascinating magnetic, transport, and optical properties, particularly at room temperature, to promising device applications. The origin of the observed ferromagnetism is also discussed, with a special focus on first-principles investigations of the exchange interactions between transition metal dopants in oxide-based magnetic semiconductors. We give a brief introduction to the oxide (ZnO, TiO2, In2O3, SnO2, etc.) - based magnetic semiconductors for fundamental material aspects through fascinating magnetic, transport, and optical properties, particularly at room temperature, to promising device applications. of the observed ferromagnetism is also discussed, with a special focus on first-principle investigations of the exchange interactions between transition metal dopants in oxide-based magnetic semiconductors.