Seems like today is murmur day huh 🙂
The murmur is a delayed because it takes a bit of time to build up the required flow velocity (and hence turbulence) to produce a murmur.
So S1 is closure of the AV, and S2 the Aortic and pulmonic valves. Diastole begins with S2 and ends with S1. So the OS should lie somewhere between S2 and S1. So the sequence you should get is S2---OS---S1. Remember also that valves open due to LA pressure rising beyond LV pressure. In MS, the LV is essentially unaffected; the volume and pressure buildup precedes it. As the LV relaxes, the MV will open once its pressure falls below that of the LA. As MS worsens, the pressure in the LA increases, resulting in earlier opening of the MV. Hence an earlier OS.
To give you some hypothetical numbers, lets say the LA pressure is 10mmHg. Once the aortic valve closes, LV pressure will start falling. Once it goes below 10mmHg, the MV will open. Now imagine this patients MS has worsened and his LA pressure has risen to 30mmHg. After closure of the aortic valve, the MV will open once LV pressure dips below 30mmHg. This will obviously happen earlier, hence the shifting of the OS towards S2.
