Teemo is definitely NOT OP.
Yes, he counters a few champions (like Garen), but (unlike Quinn and Graves skills) Teemo's Q (blind dart) does NOT actually blind anyone, it just stops BASIC attacks for 1.5 to 2.5 seconds... enemies can still use targeted skills and such.
Teemo's W skill (run speed) is nice, but the passive is only "out of combat" and the active only lasts for 3 seconds (voli and hecarim both last 4 seconds and have additional effects in addition to the run boost)
Btw - Teemo starts with one of the slower base run speeds at level one (330), which is stupid since one of his main skills focus on speed.
His E (posion) is only useful once he's buffed since it only scales with a pathetic +%30AP initial and +%10AP dot.
Teemo's ultimate, the poison 'shrooms can be annoying, but they don't stack, they must be run-over to activate, then are visible and can be destroyed for a few seconds or with a ward, and they have a delay time before they can be triggered. Sure, he'll get a few random kills from 'shrooms planted in the jungle or lanes as people flee from a fight, but although it's annoying, it's based on luck.
If anything, Teemo is actually UNDERPOWERED compared to a lot of the other regularly played champions.