Definitely not OP after a couple of matches a few thing come clean.
1.Pres 4 so you are just a click distance to get cloaked but do it when you see an enemy and when you are sure that he sees you 
than you are a tank and there is 99% chance to win the fight…
- Flank while cloaked and attack
Some will say that phantom is OP but i will say it’s even not so good as it should be . He is not cloaked he is “cloaked” it’s easy to see him in close and even mid range… yea after a couple of hours playing everyone will learn how to do a lot of kills whit it, but for players like me who are more objective base interested mercs like skyhammer, frager or even nader are more favorable choice.
So in my opinion playing phantom is really fun and all players who try it will enjoy, but still there are a lot more powerful mercs than him.