Pros:
-Aoe Slow/Speed boost
-Aoe pull
-Great At farming
-Strong auto attacks with good animation (unlike karthus)
-Strong shield.
- Amazing team fight presence with shield and Aoe CC and damage.
Cons:
- Needs blue buff or Mana built early.
- High skill cap
- Weak against Assassins like Talon and Katarina
- Weak Ganks

If you didn't notice the comment about items for Orianna Rod of Ages and Rylia's Crystal Scepter
Work amazingly If your jungler is giving you blue you shouldn't need the Athene's and if the opposing team is building armor or your having trouble surviving in teamfights Rylia's Crystal scepter could be used as your third item.

Farming: Q -> W -> E
Farming without Blue: Q -> W
Escaping Ganks: E -> W
Running late game: Q in back of chasers -> R to pull them into the ball
Chasing: Q in back of them -> R to pull them back -> W to slow while speeding yourself up
Teamfights: E initiator -> W -> Q in front of opposing carry and in back of rest of team -> R -> E shield Tank/ Low HP targets -> QW poke and protect teammates for the fight.

Highly recommended that you learn to use all these skill sequences.
Make sure to farm carefully, You can easily farm at your tower with Q -> W then autos.
Make Sure to ALWAYS keep moving while playing Orianna, I've seen so many players stand still and die while they are focused on using spells.

Command: Dissonance should be maxed first Because of it's great AP ratio And the fact that it hits enemies more often then your Command:attack.
Also Command: Protect tends to be more useful in trading because it gives a big shield that gives you bonus Armor and magic resist I recommend you max it Second

Orianna Is a high skill-cap AP mid who is very mana reliant, but she has some of the best farming abilities in the game and once you've mastered her, She can force people out of lane as well.
Her mana costs are reasonable assuming you won't spam her W you should be fine. I've never personally tried this but Orianna support can work if you believe you've mastered her but she is such a strong pick in mid lane that I don't see why you would want to. Good luck And I hope this guide works for you as it does for me evertime I play her