The flying winger spent the 2009-10 season under Postecoglou at Brisbane Roar before being snapped up by FC Utrecht, where his game has gone from strength to strength.
One of the unfortunate players to be cut from Pim Verbeek's provisional World Cup squad four years ago, there is no chance of Oar meeting the same fate ahead of the tournament in Brazil, with the 22-year-old a certain starter on Australia's left flank.
"I've always enjoyed playing under him," Oar said when asked how much he had bought into Postecoglou's game-plan following Monday's 1-1 draw with South Africa in Sydney.
"I feel free. I feel like I can try and take the game to the opposition, which is the way I like to play.
"I just tried to inject as much energy as I could and if you make yourself unpredictable with your movement then it can be difficult to mark.
“The role for Mathew Leckie and me was to try and give as many headaches as possible for defenders."
Oar was a constant menace to South Africa's backline in the first 30 minutes of the draw at ANZ Stadium, prompting him to acknowledge afterwards it was one of his best displays yet in a green and gold shirt.
"Yeah, probably," he said.
"I probably had the biggest influence on a game that I've had."
Now with 14 appearances under his belt, Oar is one of only 12 players named in Postecoglou's 27-man squad to travel to Brazil with their appearance tally in double figures.
And while Postecoglou blamed fatigue from a heavy training load for the team's second-half fade out, one of the better performers on the night offered a reminder that perhaps too much was expected from such an unfamiliar side.
"It's an extraordinarily young team," he said.
"We don't really have much experience and I think maybe that showed [against South Africa]."
