O'Neill's men go into the World Cup Group F qualifying clash at Windsor Park in buoyant mood having claimed their first win in two years against Russia last month.
And O'Neill insists that his side will need to be disciplined if they want to take something against Paulo Bento's team.
"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 again Russia."
One man O'Neill's defence will have a close eye on is Cristiano Ronaldo, who makes his first appearance since Gareth Bale's arrival at Real Madrid.
It would not be unlike the mercurial 28-year-old to produce something special at Windsor Park, but O'Neill does not expect anything different from a player who has rarely suffered from a lack of self-belief.
Asked if Bale's signing created extra pressure on Ronaldo, O'Neill said: "I would doubt that extremely. He will regard himself as the best player in the world and he proves it every time he goes on the pitch.
"His aim is to prove that every time. He is a player who has the freedom of the pitch - he's not just a threat to whoever plays at right-back, he's a threat to the whole back four.
"It's not just a case of can we get someone to stifle him in that area of the
pitch because he will just come to another area to get the ball."
O'Neill, meanwhile, has added Cliftonville goalkeeper Conor Devlin to his squad for the games against Portugal and, on Tuesday, Luxembourg.
Northern Ireland v Portugal is live on Sky Sports 4HD at 7.30pm