Darius is not OP and very easy to counter. You just have to be smart about it. He has no escape abilities, and quite squishy early game. Garen has a much stronger early game and is able to harass and push quite early, Darius needs to be a little more cautious in this regard.
You have to keep harassing Darius, annoy him. Especially as Teemo...shoot a few and run. Keep him on his toes. If he is sitting at half health, he will think twice about rushing into a fight.
He also has quite a small mana pool and it's very easy to exhaust this. Even in good circumstances, its hard to pull off more than 2 ults in succession - considering you need to stack a few attacks first. Timing the ults to ensure you kill the target to have an instant cooldown takes some skill in its own right, and normally requires Darius to take quite a beating first, as you always want to try kill a target with your ult.
I play Darius alot because he is a really fun champ to play. But believe me, early game, it can be quite scary to see how fast I lose health.
Remember keep your distance, and also remember he has a grab ability. So keep this in mind as he closes in on you.