What "Oneshot" means
Oneshot implies a single ability, or a combination of abilities all hitting at the same time, finishing the job in.. one shot. No drawn-out fights, no slow ticks down, it's just BOOM dead. Also known as an instakill.
Boards, you're misusing this term. I just saw a thread complaining about Garen being able to "oneshot tanks". ?? There is nothing in Garen's kit that allows him to single-handedly kill a tank in "one shot", unless they're the villain and already below the HP threshold that enables his ult to execute. And for a tank, that's a LOT of missing HP, and so even then, isn't a oneshot.
Please stop using buzzwords incorrectly. It really exhausts the word.