An outdoor unit for an air conditioner, comprising: a first fan coupled to a rotation shaft; a second fan coupled to a rotation shaft that is parallel to the rotation shaft of the first fan; a first bell mouth part and a second bell mouth part configured to guide air introduced into the first fan and the second fan, and spaced apart from outer circumferential ends of the first fan and the second fan, respectively; a first diffuser part having a first inner circumferential surface obliquely extending from a downstream end of the first bell mouth part to guide air discharged from the first fan, wherein an inclination angle of the first inner circumferential surface at the downstream end of the first bell mouth part varies along a circumferential direction of the first fan with respect to the axis of rotation of the shaft of the first fan and having a first opening formed at a downstream end of the first inner circumferential surface and the first opening is provided in an oval shape; a second diffuser part adjacent to the first diffuser part, the second diffuser part having a second inner circumferential surface obliquely extending from a downstream end of the second bell mouth part to guide air discharged from the second fan, wherein an inclination angle of the second inner circumferential surface at the downstream end of the second bell mouth part varies along a circumferential direction of the second fan with respect to the axis of rotation of the shaft of the second fan and having a second opening formed at a downstream end of the second inner circumferential surface and the second opening is provided in the same oval shape as the first opening; and a casing comprising a top surface, a bottom surface, a front surface, a rear surface and side surfaces, wherein the first diffuser part and the second diffuser part are disposed adjacent to the top surface, and a top plate (51, 511) is disposed at the top surface and has openings approximately matching the openings of each of the diffuser parts, wherein the oval shaped first and second openings each have two axes of symmetry, namely a minor axis and a major axis, wherein the minor axes of the first and second openings are the same, and wherein the inclination angle of each of the first inner circumferential surface of the first diffuser part and the second inner circumferential surface of the second diffuser part at a first position corresponding to the minor axis of the first and second openings is smaller than the inclination angle of each of the first inner circumferential surface of the first diffuser part and the second inner circumferential surface of the second diffuser part at a second position corresponding to the major axis of the first and second openings. a first fan coupled to a rotation shaft; a second fan coupled to a rotation shaft that is parallel to the rotation shaft of the first fan; a first bell mouth part and a second bell mouth part configured to guide air introduced into the first fan and the second fan, and spaced apart from outer circumferential ends of the first fan and the second fan, respectively; a first diffuser part having a first inner circumferential surface obliquely extending from a downstream end of the first bell mouth part to guide air discharged from the first fan, wherein an inclination angle of the first inner circumferential surface at the downstream end of the first bell mouth part varies along a circumferential direction of the first fan with respect to the axis of rotation of the shaft of the first fan and having a first opening formed at a downstream end of the first inner circumferential surface and the first opening is provided in an oval shape; a second diffuser part adjacent to the first diffuser part, the second diffuser part having a second inner circumferential surface obliquely extending from a downstream end of the second bell mouth part to guide air discharged from the second fan, wherein an inclination angle of the second inner circumferential surface at the downstream end of the second bell mouth part varies along a circumferential direction of the second fan with respect to the axis of rotation of the shaft of the second fan and having a second opening formed at a downstream end of the second inner circumferential surface and the second opening is provided in the same oval shape as the first opening; and a casing comprising a top surface, a bottom surface, a front surface, a rear surface and side surfaces, wherein the first diffuser part and the second diffuser part are disposed adjacent to the top surface, and a top plate (51, 511) is disposed at the top surface and has openings approximately matching the openings of each of the diffuser parts, wherein the oval shaped first and second openings each have two axes of symmetry, namely a minor axis and a major axis, wherein the minor axes of the first and second openings are the same, and wherein the inclination angle of each of the first inner circumferential surface of the first diffuser part and the second inner circumferential surface of the second diffuser part at a first position corresponding to the minor axis of the first and second openings is smaller than the inclination angle of each of the first inner circumferential surface of the first diffuser part and the second inner circumferential surface of the second diffuser part at a second position corresponding to the major axis of the first and second openings.