The o ring will not change the headspace if it is right. It will however correct headspace if the recess for the rim is cut to deep. If it is cut deep, when the firing pin strikes the primer, the case moves foward then the neck grips the chamber stretching the case back to the breech. If the case is resting against the breech then it just blows the shoulder out to fit the chamber, in turn leading to longer case life. Longer case life is what the op was trying to achieve.