The team go into the World Cup qualifying match off the back of their first win in two years against Russia last month.
O'Neill has said that his side will have to be disciplined if they are to get something from the match against Paulo Bento's men.
"Against this level of opposition, if you play really open and make the pitch really accessible to players of that level and ability you are asking for trouble," O'Neill told Sky Sports News.
"At the end of the day we are not going to beat Portugal 4-3, we might beat them 1-0, or 2-1. We are not going down the route of having an open expansive game of football, from the point of view that we have to play a way that gives us the best chance of winning the game.
"That is to play in a very disciplined and considered approach, and maximise the possession we get and the players that we have in key areas playing to the same level as they did against Russia."
The two sides played out a 1-1 draw when they last met in October 2012.