In Cristiano Ronaldo's 100th game for his country, Niall McGinn's strike was cancelled out by a last-gasp Helder Postiga goal and O'Neill was delighted with his players' efforts, insisting they deserved more than a point from the clash in Porto.
When asked where the result ranked among previous triumphs, O'Neill told Sky Sports: "It's up there. This is as difficult a place to come, certainly in Europe, against a world-class team.
"All credit goes to the players. They were magnificent. Their work rate, desire, the way they worked for each other - you cannot ask for more than that.
"It was very harsh to lose a goal at the end. Of course, we had to weather severe pressure but at times I thought we passed the ball well, scored a fantastic goal and we're coming out disappointed with a draw.
"The shape of the team was excellent, the work rate, the way they went about the game, was fantastic.
"To go ahead and stifle Portugal was fantastic. I'm disappointed for the players because they deserved a wee bit more than a draw from the game."
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The former Shamrock Rovers manager believes the result can act as a springboard for his side after seeing the Ulstermen drop to 117th in the Fifa world rankings.
"They have to take confidence and belief from what they did tonight," O'Neill added. "Their togetherness and spirit has been unquestionable. At times it has been questioned, but certainly tonight they answered that.
"There is more in this team, because we were down a few bodies. We've got younger players coming into the team all the time.
"If they can continue to improve and grow and obviously have more performances like tonight, then if you have that spirit you will always get something in football."