Shock wave/turbulent boundary layer interaction and flow separation may induce self-sustained large scale oscillations on a profile at transonic Mach number. This phenomenon, known as transonic buffet, is at the origin of intense pressure fluctuations which can have detrimental effects, both in external and internal aerodynamics. The present paper describes a new experiment executed in the Onera S3Ch transonic wind tunnel on shock oscillations over the OAT15A supercritical profile. These experiments have allowed to precisely define the conditions for buffet onset and to characterize the properties of the periodic motion from unsteady surface pressure measurements. The flow field behavior has been described in great detail thanks to high speed Schlieren cinematography and surveys with a two-component LDV along with a conditional sampling technique. The aim of this study was to provide the CFD community with well documented test cases to validate advanced computing methods.