As someone who plays Teemo, I'll share my thoughts. First... calling Teemo OP is silly. If you have ANY kind of gap closer, or stun, and you get to me before I can eat half your life (so, if you can get to me in the first 3-5 hits) you can squash me like a bug. Splat. Teemo Pudding. Granted.... the 3-5 hits it takes me to eat half your life takes about 2 seconds. But think about 2 seconds in a fight, team or otherwise. It's not that long. Oh, and did I mention team fights? because Teemo can be deadly if left alone fore more than a couple seconds, the other team generally focuses on Teemo. Meaning that you have 5 people doing everything they can to stun, silence, drag, root, and kill one person. Squishy as Teemo is, this usually means dead in the first half second of a team fight. It's why you'll see a lot of Teemos avoid team fights altogether.
On building Teemo... personally the On-Hit build, I find that it sucks. I'm always much more successful when I go with an AP build. Problem with an AP build is that it's really expensive, and early game doesn't give Teemo enough power to actually do the killing needed for farming. What does this mean? It means having to do /something/ to get their farming up. Usually this is when you see Teemo going with Runaan's (sp?) Hurricane. Which is something of a newb Teemo build. Matter of fact, if you have Hurricane in your build, you're likely to see your allies and enemies telling you what a newb you are. Now, you've also just spent. 2750gp to get an item that will help you get gold so you can get the items that will make you viable. Wow. That's just, awesome. Pure awesome.
Is the problem with Teemo?
Maybe, it could be he needs some of his damage to scale better with his AP.
Is the problem itemization?
It could be that as well, some of the items could use a little bit of a rework to make all Ap builds a little more viable. I mean, we've all noticed the new trend towards AD, because it's cheaper and faster. Being cheaper to build makes it faster to build. Being faster to build allows you to build early. Being built early allows you to snowball. This causes a problem for AP. because Ap builds are alll so freaking expensive.
Is the problem playstyle?
I'm going to go out on a limb and say.... maybe. I've been in teams where they supported the Teemo really well. I worked with people who warded, watched for ganks heading towards me, covered me when the other team tried to gank, and essentially let me get fat and happy, build a nice build, and use it to eat people up. It takes a lot of work, and a lot of teamwork, and a lot of dedicating the team to help one or two people get fat, but it works........ Assuming the other team doesn't realize that this is what's going on, and send their entire team to gank one person. When that happens, and they focus on person, and ignore the rest of the team, Teemo gets shut down, over and over. (See above, point about squishy).
So yeah, just my two cents.