Australia are third in Asia's Group B after two draws and a shock loss to lowly Jordan. A further defeat to fourth-placed Iraq in Doha, Qatar on October 16 would put the Socceroos in danger of missing out on reaching a third consecutive World Cup.
The 20-year-old Oar, who has four international caps and plays for FC Utrecht in Netherlands, had deserved his call-up after finding his feet in the Dutch league, Osieck said.
"We know what kind of a talent he is to offer and that he is one of the young, promising guys," Osieck told reporters in a conference call on Wednesday after naming a 22-man squad.
"He gets enough playing time (in Netherlands), enough recognition. His self-confidence is rising so therefore I called him back and I hope he will now continue his state of development."
Osieck appears set to reshuffle the defence that leaked two goals away to Jordan last month, omitting David Carney and Jason Davidson, while Sasa Ognenovski misses out due to injury.
Defender Mark Milligan has been named after serving a one-match ban for getting a red card in the Socceroos' 1-1 draw to Japan in Brisbane in June, while Italy-based midfielder Carl Valeri returns after being sidelined with injury.
Osieck will continue to rely on the usual line-up of Socceroos' stalwarts, including former Everton midfielder Tim Cahill and Qatar-based playmaker Mark Bresciano.
Mark Schwarzer, who turns 40 on Saturday, has been retained as one of three goalkeepers and is likely to remain Osieck's first choice.
"There's a way he performs, there's a way he conducts himself on and off the pitch which is unique ... You should see how hard he works," Osieck said.
The squad will convene for a training camp in Doha before the Iraq match but will not play a practice game against United Arab Emirates in the leadup due to concerns about the heat.
"Leading into the game, I need more training time," Osieck said.
Squad: Mark Bresciano, Alex Brosque, Tim Cahill, James Holland, Brett Holman, Mile Jedinak, Brad Jones, Robbie Kruse, Mitch Langerak, Ryan McGowan, Matt McKay, Mark Milligan, Lucas Neill, Jade North, Tommy Oar, Nikita Rukavytsya, Adam Sarota, Mark Schwarzer, Matthew Spiranovic, Archie Thompson, Carl Valeri, Luke Wilkshire.