In a match used to fine-tuned preparations for next month's crucial World Cup qualifier against Oman, Australia looked headed for a come-from-behind victory after taking the lead through defender Robert Cornthwaite in the 54th minute.
But strikes to Bogdan Stancu and Gabriel Torje inside the last ten minutes ensured Romania, ranked three places higher than Australia at No.33, triumphed.
Osieck made a raft of changes in the second half and felt it may have cost his team victory, but he took plenty of positives out of the match.
"When there is a couple of changes, there is always a change in the game," Osieck told Fox Sports.
"This time it was at the expense of a victory, but on the other hand I think we had so many chances in the second half and we should have finished that off.
"That is what you have to pay for if you don't score but all-in-all, I think in particular (the) second half we did pretty well.
"We showed great commitment and I think it was a very positive performance from my point of view."
With Australia's big guns all in the starting line-up, the German coach also named Cornthwaite alongside captain Lucas Neill at the heart of defence, Michael Zullo at left back and James Holland in defensive midfield.
Osieck singled out Holland for praise, indicating he was a likely starter against Oman, and was also pleased with Cornthwaite, who boosted his chances of replacing the suspended Neill in Sydney next month after his third goal in six games.
The German coach was also encouraged by lively attacking displays from in-form striker Robbie Kruse and Brett Holman, who has struggled for game time recently with Aston Villa.
Romania coach Victor Piturca restored most of his key players after they were rested in Saturday's 4-1 friendly loss to Poland and the European side was well on top early.
The Socceroos looked to be getting on level terms when they fell behind in the 36th minute.
Luke Wilkshire lost possession in a dangerous position, with Mark Schwarzer failing to keep hold of Torje's shot and Christian Tanase firing in from the rebound.
Then Kruse had a chance to hit back almost immediately when he broke on goal but he scooped his shot over the bar.
But Australia pulled level on the stroke of halftime through a Wilkshire penalty.
Holland played Alex Brosque into space in the box where the striker was clattered by Razvan Dinca Rat.
Adam Federici replaced Schwarzer just after halftime while Osieck also injected Ryan McGowan off the bench in place of Wilkshire.
Australia took the lead in the 54th minute when Neill headed Matt McKay's corner back into the box and Cornthwaite showed good determination to fire the ball home on his second attempt.
Holman squandered an excellent chance to extend Australia's lead in the 65th minute and Federici denied Stancu with a great save in the 73rd minute.
But Stancu made amends when he levelled the scores in the 80th minute, tapping in following some good lead-up work from substitute Alexandru Maxim.
Romania netted the winner just three minutes later, with Federici flat footed as Torje sent in a brilliant free kick after substitute Rhys Williams had fouled just outside the box.
Nikita Rukavytsya, Matthew Leckie and Mark Milligan all saw time off the bench in the second half.