Most of the champions that are considered "op" right now can actually be countered fairly easily, but people simply don't build against them or work to beat them. Most of the times they are champions that only become a huge problem if left unchecked (like kog maw)
Kog maw has a notoriously weak early game. Crush him then. Also, as soon as you see the other team picked kog maw, react by making sure your team has some lockdown potential - Someone like nautilus, or leona, or malphite, or any cc machine. etc. Someone that can shut him the fuck down, he is SO reliant on his team protecting him from cc, so get something he can't be protected from.
Master yi's another one I've seen people complaining about recently. He's another perfect example - people think he's op because they let him farm all game and are surprised that, when he gets sated, he slices and dices. COUNTER jungle him, he is very weak early and extremely susceptible to being buttfucked while clearing his camps. Take advantage of that, and making sure your team has some hard cc is good too. He slices and dices if left unchecked, but if he gets stunned he will fucking melt quicker than ice cream in an oven.