var lastWorldOrientation = getHMDOrientation();//This method returns world space orientation
var displayedOrientation = lastWorldOrientation;
void Update() {
    var nowWorldOrientation = getHMDOrientation();
    var delta;

    if (lockedYaw) { //null if not locked, else float with value yaw locked to
        var tempLast = lastWorldOrientation;
        var tempNow = nowWorldOrientation;
        tempLast.eulerAngles.y = lockedYaw;
        tempNow.eulerAngles.y = lockedYaw;

        delta = tempLast:toObjectSpace(tempNow);
    } else {
        delta = lastWorldOrientation:toObjectSpace(nowWorldOrientation);
    }

    lastWorldOrientation = nowWorldOrientation;
    displayedOrientation = displayedOrientation * delta;
    camera.rotation = displayedOrientation;
}