A lot of people giving good explanations here but I?ll throw my 2 cents in just in case your brain is on the same wavelength as mine as far as how to understand it.
A player being behind the defense when they receive a pass is not automatic evidence of offsides.
Say a defender and the receiver of the pass are right beside each other (the receiver is not offsides). The passer is allowed to LEAD the receiver. The passer can kick the ball to a point on the field behind the defense. If the receiver is then able to simply run faster than the defender, and get to the ball first, that is fair game (which is exactly what happened on this play). The receiver is just not allowed to sit and WAIT behind the defense before a pass comes. But as long as the receiver is onsides when the pass is INITIATED, he is free to then beat the defender to the ball. Pannholzer was onsides when Janics foot contacted the ball, which is all that matters. Then Pannholzer got ahead of the defender while the ball was in the air. That is totally legal.
Edit: Not sure if you were asking about Pannholzer being behind the defender when he received the pass from Janic, or if you were asking about the fact that Pannholzer took the shot after he got past everyone on the defense (including the goalie), so I will address that too:
Once he has the ball, it doesnt matter. Simply being behind the defense is not illegal in and of itself. And there isnt a rule that a defender must be in front of you in order to take a shot on goal. Thats not what the offsides rule is about. Its only a matter of waiting/hiding behind the defense to receive a pass, and therefore not giving the defenders a fair shot at stopping you.
The player with the ball is allowed to take the ball himself and run around the defenders and then shoot a goal, because the defenders had a fair shot to stop him and he was just faster or more skilled. The OSU defender and goalie had a fair shot to stop Pannholzer, but he just straight up put the moves on them. There is no rule against scoring a goal after beating the defenders by running around them with the ball, as long as you legally received the ball in the first place. The offsides rule only has to do with passing, and your position relative to the defense when a teammate passes you the ball. The man with the ball can never be offsides, as long as he received it from a legal pass.