In this study, the wall-modeled large-eddy simulation (LES) of transonic buffet phenomena over the OAT15A supercritical airfoil is conducted. The computational results are compared with zonal detached eddy simulation (DES) and experiment. By using the wall-modeled LES, the buffet onset is successfully predicted. The small separation near the trailing edge is also accurately estimated with buffet phenomena. Velocity properties show good agreement with experiment except the region in which the shock wave moves. From the trace of the shock wave movement, the buffet frequency which nearly equals to experimental value is obtained. The self-sustained oscillation mechanisms are investigated from the computational results. As a results, Lee’s acoustic feedback loop model cannot predict the buffet frequency. On the other hand, the possibility of the proposed model is confirmed.