The midfielder played a starring role in the 2-1 win at Dagenham and Redbridge on Tuesday and the Chairboys followed that up with a 1-0 win against Exeter City last week.
But despite those two wins, O'Nien believes it will be a tough test against the league leaders at Sixfields (3pm k.o.).
He said: "They are going to be the favourites but we can go into the game with nothing to lose.
"We will give it our all and hopefully at the end of the 90 minutes we will have a good result."
Northampton come into the game having won their last eight games on the bounce, including a crucial 1-0 win at promotion rivals Oxford United on Tuesday night, but Luke feels that form goes out of the window in big games such as this.
He said: "The last few games have been really good for us and we got six points from the two games last week, but it counts for nothing going into this game.
"Even if you're in a good run of form at the end of the day it's eleven against eleven. We're all looking forward to it so it should be a good game."
The reverse fixture saw the Cobblers win 3-2 back in October, a game which Luke scored the opening goal.
"I scored but we lost so I would rather win the game and not score!" he said. "If I can score and we can win it would be a massive bonus but as long as we get the three points that's the most important thing."
Watch the full interview with Luke
, as well as his thoughts on the club's Professional Development Phase
scheme, in Chairboys PlayerHD.