After opening their campaign with three straight victories, Northern Ireland fell to a first defeat and surrendered top spot in Group F on Friday as Paul Papp scored twice in the second half.
However, with four of their next five fixtures being at home, O'Neill remains hopeful of eventually booking a trip to France.
"We now have two home games to look forward to and have nine points from four matches, three of which were away to the hardest teams in the group," said the Northern Ireland manager.
"It's a fantastic start for us and we'll get ourselves ready for March. I don't think we could have had a more difficult start to this campaign and we now have to use that platform in the home games.
"They'll be difficult too, but games five and six are both at home, with the opportunity to get to 15 points. If we can manage to do that, we'll be in a very strong position."
Northern Ireland were without captain Steven Davis, Shane Ferguson and Jamie Ward and O'Neill admitted that tested the strength in depth of his squad.
"I couldn't have asked any more of the players in terms of their commitment and work-rate," he added on Sky Sports. "We know we can play a bit better, but the players we had unavailable hurt us.
"We defended fantastically well for long, long periods and possibly could have done a little bit better with the chances we had to counter-attack.
"It's disappointing to concede the goals the way we did, but we still came within 15 minutes of getting a result.
"Romania are a very strong team, though - the strongest in the group and there's no doubt the best team won on the night."
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