It's also doubtful because the vestibule door was very thick and solid specifically to stop sound getting through. Also, if there was a flinch it would surely have occurred when Truly came up the stairs first, Oswald would be out of sight by the time Baker came into view.
Time for some unprovable speculation - maybe Oswald had finished his lunch and was about to exit through the vestibule door when he saw Truly making his way across to the flight of stairs leading up to the third floor. For some reason I'd have to completely make up he decided he didn't want to be seen, turned and was spotted by Baker.
To be honest I'm starting to have serious doubts about the whole lunchroom confrontation.