Since Riot has a ****** for generic bruisers and doesn't release any supports anymore, I was forced to always pick the same ******* characters. Now this is absolutely acceptable if you own pink Taric, but still a bit dull. So I went and tried to support with something else than the traditional top-picks. And Orianna works well as support, at least for me. But lets see some pros and cons, then we can decide if she's viable.

Pros

Ball can be used to scout
Great CC
Shield and Armor/Magic-Resistance-Buff

Cons

No Heal
Bad poke/harass
Requires some practice

Viable?
Her unique quirk is, of course, the ball. It's totally awesome for scouting- I'll get into the details later. You may think that I'm exaggerating when I talk about "Great CC", since she seems to only have a slow besides her ultimate, but dont forget that Command: Dissonance has a build in AOE-Speedbuff. That's really ****ing useful and totally beats Sonas ****ty speedbuff. Her shield can be a little tricky, and requires practice, but offers some Armor and Magic Resistance. Now her main disadvantages are her bad harass- actually, it is possible to play agressively with Command: Attack, but that leaves your carry open for counter-harass and drains lots of delicious mana.

So, is she viable? I wouldnt consider her on par with the traditional top picks, but **** that. She's fun and definitely worth playing if you aren't betting your soul on the game. That said, her kit is kinda like that of a weaker Janna, so she doesn't fill a niche like Nidalee, which works as support because of her awesome poke. Just try her in some normals, and decide for yourself.

Clockwork Windup
Don't underestimate the damage it adds to your normal attacks! Since you can't really harass with your ball, do it with standard attacks.

Command: Attack
Get a feeling for the time the ball needs to travel. It will make the difference between a lost and a won teamfight, since you can only influence the area close to the ball. Also, use it to scout, whenever you can. It might save your life.

Command: Dissonance
Aww, **** yeah. It's a speedbuff and a slow. In the same ******* spell. Isn't that awesome? Use it on yourself, if you want to get somewhere quickly, use it on your lanemate, if he needs to escape a gank, use it on your tank, if he wants to engage. Always remember, that the area stays for a while, so position it in the way of as many teammates or enemies as possible. If you shielded someone with Command: Protect before, you can quickly follow up with Command: Dissonance.

Command: Protect
Always keep the ball on your AD-Carry while laning, except for scouting. The shield pops up when the ball reached the target, which means that you can only effectively protect it from burst if the ball is close. This means, that you cant really harass with your ball, like I said before. The shield, however, persists even if you move the ball after it reached its target, so throw it in the enemies face after shielding your carry. Gives a nice Armor/Magic-Resistance-Buff as well.

Command: Shockwave
Did someone say great CC? This ultimate wins teamfights. This, again, is a skill that needs some practice. Not only do you need to predict the movement of the enemy, but you also might end up pushing the enemy in the direction of your injured carry. This is, while funny, not the wanted result. So yeah, get a feeling for this strange spell. Also, use it in combination with Flash, to reduce travel-time of your ball.

The first point goes into Command: Attack, because your ball is quite useless if you cant move it. Scouting and ****. Since Command: Protect scales nice I max it first. After that I prioritize Command: Dissonance for the slow and speedbuff. You might want to take a point in it at level 3, if you play against a dangerous jungler like Shaco, or your ******ed lanemate pushes too hard. Ah yeah, take a point in Command: Shockwave, whenever you can. I dont even have to say that, do I?

Useful for repositioning, escaping, even with the recent nerf. If you have the ball, flashing and then throwing it reduces, of course, the travelling time of it. Using that in combination with Command: Shockwave sometimes wins teamfights. So get a feeling for when it is appropiate to use Flash offensively.

Thats really a standard support pick, even after the nerf. Map awareness wins games.

The buff made it viable again, and it might save your stupid AD-Carry. Take it if you aren't comfortable with the nerfed clarvoyance or dont need Flash. (Because, as we know, your positioning is always perfect!)

I choose life over Magic Penetration because Orianna needs to keep the ball close to her carry, which leads to a pretty defensive style of play. But Magic Penetration also works fine, since Clockwork Windup adds TONS OF DAMAGE to her auto-attacks. Other than that, pretty self explanatory, really. Like masteries, this is personal preference. Do whatever you like.

Stamdard support stuff. Since Orianna is a bit mana-hungry I start with a Nomad's Medallion. Then, of course, Boots. Since she scales well with AP I throw in a kage's lucky pick, You might want to throw in a Heart of Gold, if your team is behind. Ionian Boots of Lucidity or mercury threads finally gives you some more movement-speed. If your tank doesn't build it I recommend an Aegis of the Legion. I build it a little late, but it's still a really good item, which offers benefits for the whole team. After that squeeze in a Shurelya's Battlesong because the active isAWESOME.

If your team is balanced, I recommend to get a Will of the Ancients last, it not only benefits your whole team, but finally boosts your AP a little, which is always welcome.
Alternatively, if your AD-Carry is getting Pentakills, go for a Zeke's Harbinger, even if it doesn't really benefit you directly. Because as the support you're just some ***** who has to do everything the ******* carries don't want to. That's right, I said it. You have to spend your whole monies on wards, can't farm and don't even get kills. But yeah, if the jungler ganks your lane, after your Level 3 AD-Carry towerdived it's your ******* fault. Whatever, just fill that slot with something which benefits your whole team. If you played support a while, you should know what I mean.

Don't forget to buy sight wards, Vision Wards and Oracle's Elixirs.

Laning
Don't take creeps if your AD-Carry is on the lane, you're support after all. Use your ball to scout suspicious brushes, otherwise keep it near your carry, because- I already said that, three times. Not gonna do that again. Harass the enemy with Clockwork Windup, like a bauws. Don't forget to buy wards, so you can laugh at the pathetical attempts at ganking from the enemy jungler. Generally, you're better at defending your carry, than harassing the enemies. So don't get cocky, and only harass if your lanemate doesn't need the shield- which allows you to eat some counter-harass. Also, if some annoying enemy sets up his camp in some brush, just keep the ball there. They will think about their positioning twice after your AD-Carry raped their ****.

Teamfights
Throw Command: Dissonance in front of your team if they initiate. Priority target for your shield is the AD-Carry, then the AP-Carry. If you haven't wasted your ultimate yet, you. Then the rest of your team. Use Command: Shockwave in the first 5 seconds after the engage and try to throw their carries together, so that your AP-carry can burst them down. If that doesn't work, wait and try to hit the AD-carry and as much enemys as possible.

Support Duties
Don't forget to ward, and remember the respawn times for Dragon/Baron.

Yeah, well, this is quite a short guide, but I hope it explained the unique mechanics of Orianna in regard to playing her as a support. I in no way intended to teach the reader anything about the support role in general, so please excuse me for the lack of explanation in some paragraphs.
In my opinion support Orianna is great fun due to her unique playstyle. But don't expect a top pick. With some skill, however, you should be able to compete with most traditional support characters.