Liverpool youngster Conor Bradley was captain for the tie, which was held in Hungary.
"I'm frustrated and slightly disappointed - but only with the scoreline, to be honest," O'Neill told BBC Sport NI.
"First half we played really well, were very dominant and played some really good football.
"We just needed that first goal, then the pattern of the game might have changed a little bit and there might have been more space for us."
He added, "We are not going to be a high-scoring team at the minute.
"It is always a concern when you are not scoring, but it is not something we have not had before, and we just need to find ways to win.
"We have had three games in the Nations League and two clean sheets.
"I tend to find that with a young team like this, that there are learnings.
"We kept a clean sheet, we weren't beaten, and the shape of the team was good in and out of possession, and a lot of our football was good."