O'Neill believes that Northern Ireland still have a lot more to offer after their famous result.
"They have to take confidence and belief from what they did tonight," said O'Neill. "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."
The match marked Cristiano Ronaldo's 100th cap for Portugal.