You don't get an OA because someone makes an attack, so that's silly and stops right there. You can't just invent new rules when you want to like making an OA when someone attacks you.
But, even if you could, the OA isn't granted by the spell, so even if the spell triggers it, it's clearly not granted by the spell like an attack using the action granted by haste is provided by haste. Your argument boils down to saying 'the spell gives you a great big bundle of choices, but if you pick one of those choices, they're not granted by the spell, because the spell only grants the bundle.' If I give you a bag with either a snickers or a butterfinger inside, but tell you I'm not sure which is in there (I've given out a lot of bags, maybe), when you reach in and pull out a butterfinger, I still gave it to you, the bag didn't step in as a magical middleman and make the butterfinger come from someone else. The same with haste. Just because it involves a choice in what you take out of the bag haste gives you doesn't change the fact that it came from the haste. You can't say 'haste gives you a choice, but when you choose then haste has nothing to do with that.' You've arbitrarily inventing a cut-out that doesn't exist anywhere.