I'm not so sure about O'Mahony in a midfield role. Definitely worth trying there in the league, but to my mind he is a more limited player than some people suggest. I have never seen him as the kind of player who scraps for dirty ball or holds up opponents in the tackle. His concentration and positioning was a problem when he played in defence, and that might also be a factor at midfield. And I still think he tends to take big swings, making him vulnerable to being hooked. He brings big positives in terms of running with the ball and shielding it from his marker when he gets out in front to win it, so I agree that there should be a place for him in the front eight. But I also think he'll need a fair bit of coaching to smooth off the rough edges to his game.
In the league, I reckon he should get a game or two at midfield with O'Sullivan or Roche, but also spend time at wing-forward or FF.