Right, my first guide yet, and I created it, cos I haven't yet saw the right build for lady Orianna. She is not tanky, she isn't even sopposed to BE in teamfights, she's not in the fray, she's in the back shielding slowing and dealing big aoe damage to the enemy team. That's why i prefer building her STRAIGHT ap, and stacking Archangels gives you the mana, to keep spamming and spamming your "Ravage" over and over doing dissonance as often as possible. That's the way you play her my way.

It actualy depends on the situation. If you want to harass alot, you can take your Command attack, if you want to deal a bit more aoe damage, but sacrifice your mana to it, you take dissonance, the shield, i don't admire so much, I prefer the damage output over the support, but than again it's your choice, depends on gameplay in my opinion.

I take flash, as it is one of my favorite spells, but ghost.png;2 ghost is also viable, she needs the survivability and flash (or ghost) provide it well. About the second spell, I've tried many:

Clarity - you have runes for that as well as archangels, but if you really need it, be my guest

Clairvoyance - your command attack reveals the brush, so you can use it as a short range ward, still usable in different lanes, or to see baron, feel free

Exhaust - you have dissonance, but it is viable instead of ignite

Heal - if you're a fan, feel free

And finally Ignite, extra firepower to pick up the kill, so far my favorite of the secondary, great on taking on those lifesteal tanks as mundo, swain, (even) master yi.
The rest of the spells, are more or less useless in my eyes, but if you like, you can take them

So you say if you get that rod of ages, rylais and what not that gets you some health, what use are you to the team? You plan on rushing into the massacre against the enemy xin, hoping you can solo him? Well, I'm not sure if that's the best idea. I managed to drag xins, olafs, master yis, even ashes behind me while straight raping them till they die. So it works, trust me, takes practice though, you have to know when to hit dissonance, attack and protect, but it simply comes to you as you play. Practice makes perfect.... That's not what i meant to explain. About the items again. If you get jumped on, the survivability doesn't really add much. You're dead unless you flash and dissonance. And after spamming those skills, in a while you'll see that you are mana dependant. That's why archangels are my choice, they give a great AP boost as well as the nessecary mana.

Pros:

You have a ball.
Great harass abilities
Nice damage output
Supportive- shield,speed/slow
You have an amazing ball
A nice interrupt ultimate, that has many uses, so use it wisely (it's easy to get ****ed with it by pulling that evil yi closer to yourself)
Did i mention you have a ball?

Cons:

Squishy
Mana hungry early
Takes a lot of practice to master her ball movement
Takes mana to farm minions
Can't think of more now, if it comes to me, I'll update this

Ah ,Orianna, she is a great damage dealer, as well as a nice support Champion. She's extremely fun to play and she has a ball... Can it get any better?! I wouldn't recommend her for a new player, you might want to learn how to play first.