Macclesfield sit bottom in League Two, meaning they are the lowest ranked side in the Football League, but O'Hara is counting on his penalty-saving expertise and tips from David de Gea to help the 'Silkmen' achieve a result at the London Stadium.
The Republic of Ireland under-21 international saved a hat-trick of penalty kicks in the last round against Walsall, and insisted he is eyeing another star performance - this time against Premier League opposition.
"I'm not a big fan of research. You can look at how many penalties someone has taken but on the night he could put it anywhere.
"You have to try and get a read as they are running up to the ball, their body language. You see a lot of goalkeepers go early and the taker just waits and puts in the other way.
"If you hold your position as long as you can and get a step in, you're going to get there. I'm quite quick."
The 22-year-old also admitted he feels he has improved from training alongside David de Gea at Carrington.
"Every goalkeeper has their own style but there are definitely things I've seen from David that have made their way into my game.
"He makes a lot of saves with his feet and close-range blocks and I'm similar in that respect. He's unorthodox but effective in keeping the ball out of the net.
"You see a lot of goalkeepers trying to get their hand to the ball and they don't make it. David realises early he's not going to get there with his hands, sticks a foot out and makes a world-class save."