@.access:
There is also the possibility that the evil brother, being able to steal a person's quirk, can somehow sense what ability someone has. This way he could know his brother's quirk and planned to make him harbor OfA until one day he could steal it for himself.
Although I find more likely that he gave both abilities to his brother. Less dependent on amazing coincidences (although less romantic as well).
Thanks, I was just about to mention this. I find the idea that the Villain created OFA intentionally both likely and compelling, but in order to be accurate one of either two things needs to be true:
Either A) the Villain had a way of detecting and identifying quirks so he knew of his brother's quirk in advance. He then grants stockpiling and plays the odds knowing his brother's sense of justice and the possibility of quirk melding.
Or B) the Villain gave both abilities to the brother and the brother (or someone later down the line) lied about it to the next wielder of OFA. The Villain is somewhat less manipulative in this scenario and the OFA line bears more guilt.
So either the Villain has a way of identifying quirks before he takes them or one or more OFA ancestors were aware of the Villain's plan at some point in the past.