Oswald is having his lunch on the second floor, he hears a muffled noise and decides to investigate. If its someone coming down the stairs or a distant voice why would he bother, neither are unusual enough to concern him. If its gunshots he would be stood at the vestibule window for over a minute. And why not just go out to the stairs if he's so bothered? And why not flinch from Truly but flinch from Baker. It's all unfounded speculation that arises from the fact that if Oswald is in the lunchroom Baker doesn't see him.
You've probably gone through this many times but what's the compelling evidence that has Oswald in the lunchroom when Baker and Truly pass by?