An oxide-based solid electrolyte according to the present invention may be LixLa3M2O12 and may have a cubic phase. The oxide-based solid electrolyte may further include first and second dopants. A method of preparing an oxide-based solid electrolyte according to the concept of the present invention may include mixing a lithium compound, a lanthanum compound, a metal compound, a first dopant precursor, and a second dopant precursor to prepare an intermediate, and crystallizing the intermediate to prepare LixLa3M2O12 crystals having a cubic phase.