User blog comment:ElizabethVictoria00Catasthrophe/Izuku is not really quirkless/@comment-35975040-20180713094957

I don't think he was able to control it at all. It just exploded whenever he needed it. The first time he truly "controlled" it was when he created his Full Cowl technique during his time with Gran Torino. Additionally, All Might taught him how to access it. As far as I know, One For All works like a passable power source, and the users store power and access it.