OK, I see that now. How about an example of an interrupt that invalidates the OA and is an attack?
There are various interrupt powers that prevent one being hit. Disruptive Strike, a ranger attack that applies a penalty to hit. Sudden Scales, a sorcerer utility that gives a large bonus to defenses. Wizard's Escape, a wizard utility that lets you teleport when hit.
Now, this doesn't make it too abusive to use this to negates OAs. They are designed to negate attacks - in this case, you are negating an attack on your turn instead of one at some other time. It adds some complexity to things, yeah, but is doable.
It does get tricky with some of these, that let you shift or teleport or the like - basically, by the enemy taking an OA on you, you can eke out some extra squares of free movement. That might bother some folks. Still, not the end of the world.
But it can get more complicated. This is actually an example from a game I'm in, and something one of our PCs wanted to do - but couldn't, because of the rules against immediates and OAs on your own turn.
He is a beastmaster ranger. He has the feat Beast Protector - if an enemy makes a melee attack against his companion, doing so provokes an opportunity attack from him.
So what he thought he could do was move both himself and his beast, and have his companion provoke from an enemy. When that enemy takes an OA on the beast, it provokes an attack from him.
Now, that might give him free attacks, but it may not be abusive - he wasn't really trying to break the system, just trying to make good use of his feat.
But what if we go a step further? Let's give him a hammer and the feat Hammering Iron - when he hits with an OA, he can push the target 1 square. Now, he not only gets to punish the enemy for hitting his companion, he gets to push them and prevent their OA from happening at all.
Still not game-breaking - the opponent can always choose to just not take the OA, of course. But still - this is the sort of thing you can set up, ways to make taking OAs (on either you, or allies, or companions) just a bad idea for them.
As others have said, if your group isn't going out of their way to abuse it, changing it can be fine. Just be aware that it
does allow some potent combos, not to mention it could trigger some quite complicated scenarios.