Smith-Purcell (SP) radiations of weakly relativistic oversized backward wave oscillator (BWO) are examined experimentally. A bunching section composed of a coaxial slow wave structure with inner K-band corrugation is placed upstream from the oversized BWO. An annular beam less than 100 kV is self-bunched in the bunching section and then excites the oversized BWO. SP radiations are examined by using 25.5- and 45.0-GHz oversized BWOs. The beam is too far away from the walls of the oversized BWO for SP radiation to occur without the self-bunching. When the corrugation parameters of the bunching section are the same as those of the oversized BWO, enhanced SP radiations in the frequency region above the surface wave are observed even when the beam propagates outside the interaction region of the surface wave.