Regular Olyroos representative, goalkeeper Jack Duncan, told The World Game the pair had made a huge difference to the team in their second game of the group stage in Doha, in which they beat Vietnam 2-0 to keep their hopes of advancing to the knockout stage alive.
The points table and the goals for-and-against differences make the equation simple for the Olyroos, who are currently third in their group on three points - one behind both Jordan and UAE.
They must beat Jordan in their last group game (3.30am Thursday AEDT) to guarantee that they continue in the tournament. A draw would only be good enough if Vietnam managed to score an unlikely upset win over UAE.
Duncan said Melbourne Victory defender Deng, just 18, and Sydney FC defensive midfielder O'Neill, 21, had shown maturity well beyond their age and maturity as the Aussies fought to get back into the reckoning in the AFC U23 Championship.
"Tommy and Brandon didn't have the experience at this level that many of the other players had, but either of them could have been man of the match against Vietnam," Duncan said. "Overall, I think we got it just right in that game.
"I've known Brandon for a while. We were at Perth Glory together and I know the type of player he is and he's been doing really well for Sydney in the A-League this season.
"He brought what he has learned into our team for the game against Vietnam. His composure and his technical ability on the ball when put in tight situations was outstanding. He really helps our ability to get the ball from a build-up out of defence into attacking situations.
"He's a very physical player and I think that's the Irish in him. He's hard to push off the ball and he always works hard to win it.
"He's got that grit and determination and he knows his role as a defensive midfielder is not just to look pretty on the ball, he's got to get stuck in and he does that job really well. He started a lot of our attacks."
Duncan said Deng was very calm in pressure situations for such a young player.
"He came in and put in a really solid performance, his composure on the ball was outstanding," Duncan said. "Vietnam was always going to press us a little more than UAE did in terms of trying to stop us from building up from the back and I can't praise him enough for how well he performed.
"Tommy has put in some big performances early in his career for Melbourne Victory and now he has made a great start for us as well."
Duncan said the result against Vietnam had lifted the mood in the camp.
"There's a lot more energy and a lot more positives to consider," he said. "We worked the ball a lot quicker against Vietnam. We changed a few things going into the game and identified where to push at Vietnam's structure.
"We got that spot-on because we were able to play through them and find little gaps really well."
Duncan said it was better for the players that they know exactly what result they need against Jordan to get through, rather than be unsure because of a myriad of mathematical possibilities.
The top three teams in the tournament advance to the Olympic Games in Rio de Janeiro in August.
"After the first game we looked at the two games left as cup finals - both must-win games," Duncan said. "Our mentality is that it's black and white, we know what we have to do.
"We'll go into this last game knowing that a win will give us what we need to progress - possibly as the top-placed team but if not then certainly as the second-placed team."
