The 27-year-old Hull frontman gave Ireland an early lead against Serbia in Dublin, but then passed up two glorious further chances as they slipped to a 2-1 defeat - the first defeat of O'Neill's reign.
O'Neill said: "I think he is frustrated with the fact that he could easily have scored a hat-trick, and the two chances that fell to him, considering the first goal he scored was actually on his left foot, the other ones I think he thought he could have scored.
"It didn't happen. Maybe those might go in on the big occasion eventually when he is called upon.
"But in terms of his contribution, where he wants to try to improve is the overall ability of a centre-forward to be able to take it in, keep the ball for a second or two and bring other players into the game.
"But I think he is attempting to do that and he is trying to improve on that."
The Republic, who had seen a second-minute Wes Hoolahan strike harshly ruled out for offside, took the lead six minutes later when Long capitalised on an error by Branislav Ivanovic to beat keeper Vladimir Stojkovic.
However, Long could not repeat the dose on the half-hour after the impressive Hoolahan split the visitors' defence wide-open, and missed again from the same source eight minutes after the restart.
O'Neill said: "It was a terrific ball by Wes Hoolahan, an absolutely terrific ball to put him in [for the first chance].
"With his confidence being high, I thought he might have taken that another yard or two and put it in."
Having levelled through James McCarthy's 48th-minute own goal, Serbia won the game on the hour when striker Filip Dordevic stabbed home from close range.
Asked about Hoolahan's contribution, O'Neill said: "I was very pleased, not only with him, but a few others as well. We started very, very brightly in the game had a goal which was disallowed.
"It was disappointing to find out it was about a yard and a half onside. I didn't know that, otherwise I would have gone mad.
"The referee, I have just mentioned it to him and he has apologised if that was the case when he sees it. It's disappointing, that, really because he was well onside.
"Having said that, we scored a very good goal and we had a great opportunity to go 2-0 in front and I suppose at international level - I suppose at any level - that would have been a big moment for us.
"We had done very well up until half-time, but we started a bit hesitantly in the second half, allowed them into the game.
"They scored a goal and we lost our way a little bit then for a while."
Opposite number Ljubinko Drulovic was understandably delighted with the resolve his players showed to recover from a bad start to the game.
He said: "I'm very happy and very proud. We still have a long way to go, but we played against a very strong team in the Republic of Ireland and I send very big congratulations to all our players."