OpsKT
Explorer
99% of the time, Reach doesn't apply to OAs. Reach only applies on your turn, unless you (or, much more commonly, the monster) have "Threatening Reach".
Currently, there is no way to get Threatening Reach on a PC that I'm aware of. I may have missed one, but Polearm gamble does give an Opportunity Action upon them entering, at the trade off of giving the enemy Combat Advantage. But that's not Threatening Reach, that's more like 'set vs charge'.