In this study, a wall-modeled large-eddy simulation (WMLES) of transonic buffet phenomena over an OAT15A supercritical airfoil at high Reynolds number is conducted. By using the WMLES, the buffet onset is successfully predicted. The small separation near a trailing edge is also accurately estimated with buffet phenomena. The flow physics of buffet flow are investigated from the results. As a results, it is confirmed that there are dominant phenomena in low and high frequency band. By using the phase average operation, large-scale vortical structures with low frequency band and acoustic waves generated from the trailing edge are identified. At last, we propose the new self-sustained oscillation model which depends on the pressure ratio forward and backward of the shock wave. The possibility of the proposed model is confirmed from the results.