It was Parrott's heroics which set up Ireland for their World Cup playoff tie with the Czech Repulic.
O'Shea said of the AZ Alkmaar striker: Troy's reaping the rewards of the practice and dedication he's given to himself and his career over the past few years.
"You've seen other strikers have plenty of loan spells in English leagues and it takes a few spells for everything to click in terms of the physicality and mentality side of a player developing.
"Game understanding was always there with Troy, even from that underage capacity he had for scoring goals.
"It was just a case of knuckling down, doing the hard yards, doing the work, making sure he was staying as fit as possible and being clever enough to pick his club, he's playing in Europe, playing that level.
"He's been very clever and calculated about where he's gone to develop."
Brentford captain and Ireland defender Nathan Collins also said of the former Tottenham centre-forward: "In football, sometimes, it doesn't click on the pitch. That doesn't take away from being a good footballer, from understanding the game or knowing how good he is.
"There are loads of different variables and reasons as to why he might not be clicking at that time.
"But there has always been a footballer in Troy. I am so happy now that it is shining and showing how good he is."