A compatible optical pickup including an optical unit and a focusing unit. The optical unit emits a short wavelength light for high-density recording media and a long wavelength light for low-density recording media, directs the short and long wavelength light to a recording medium, receives lights reflected by the recording medium, and detects an information reproduction signal and/or an error signal from the received lights. The focusing unit focuses light received from the optical unit to form a light spot on a recording surface of the recording medium, diffracts the short wavelength light into a zero order light and the long wavelength light into a second order light to be used as effective light for recording and/or reproduction. Thus, a high-density recording medium and a low-density recording medium having different thicknesses can be compatibly used.