A motor engine having an oxygen sensor is provided, including: a cylinder head, including a main body and a protruding tube protruding outwardly from the main body, the main body including a first end portion, a second end, and a combustion chamber, the protruding tube having a protrusive receptacle protruding outwardly therefrom, the main body defining an axial direction; an exhaust passage, communicated with the combustion chamber and the protruding tube; an oxygen sensor, inserted into the protrusive receptacle, including a sensing portion which is inserted into the exhaust passage; wherein as viewed in a lateral direction of the cylinder head, two surfaces of the main body along the axial direction disposed on two opposite sides defines two boundary lines, the oxygen sensor and the main body are in an overlapping arrangement and does not protrude out of the two boundary lines.