Look at the RC p246. Obviously what happened is some nimnuts somewhere was told "reformat things as powers". Unfortunately someone went WAY too far with that. They formatted the triggering of an OA as a totally unneeded 'power'. It is an Opportunity Action, with Melee 1 range and the usual OA trigger condition. Its effect is that you get to make an MBA.
This is because the trigger for OA involves adjacency, in other words, IS of melee 1 range.
You see the problem here.
Nope.
The "Opportunity Attack" power isn't using the reach weapon.
Of course not, because it has no parameters that a weapon plugs in to.
Even if your theory was correct, which I am not sure it is, it is moot because Opportunity Attack doesn't have a weapon keyword at all! In fact it has no keywords.
It doesn't need keywords. It never did.
It really is absolutely not something that should be a power at all and can be safely ignored. But so stands RAW, officially it is IMPOSSIBLE to have an OA beyond 1 square unless some other game element actually specifically overrides p246.
This is true.
All the stuff that did allow it when OA was just an MBA no longer functions by RAW.
Except this is not.
See, everything that changes how opportunity attack works is
specifically naming opportunity attack, and therefore is a specific exception to the general rule.
For example, Polearm Gamble specifically changes it so that an opportunity attack can be delivered to a non-adjacent creature and adds a different trigger that allows it. By naming opportunity attacks, it changes how opportunity attack works. It's still a contradiction of the opportunity attack power, and in the case of every rules contradiciton
specific beats general.
Threatening Reach changes the range of opportunity attacks by allowing them against non-adjacent targets. It does not matter what range the opportunity attack power has, as by contradicting that,
specific beats general.
Neither of those elements only mention a 'provoke range' or 'attack range.' They say you can use opportunity attacks against those creatures. That means that
FOR ALL PURPOSES you can do so. It does not specify that only subsets of the opportunity attack rules work, and I'm completely baffled as to how one could come to that conclusion.
These things work because they
say they do. Specific beats general allows this to happen until all contradictions are involved.
The only way it wouldn't work is when you have two equally specific rules clash. This is not the case.