A high frequency, early to middiastolic sound, heard after S2
Occurs when mitral leaflet motion suddenly stops during diastole because the stenosed valve has reached its maximum opening
A shorter interval between S2 and opening snap indicates more severe disease; occurs because left atrial pressure is greater than left ventricular end-diastolic pressure (LVEDP).
Isovolumetric relaxation phase takes shorter to reach equal pressure with atrium.