Try your best to not get hit.
Orianna has amazing harassment capabilities, use them well.
Do not solo with this build, Orianna needs someone to be with her and help her set ganks
This build is primarily for Summoner's Rift... hints the Neutral Buff mastery.

You can switch the Quintessences of Potency (flat ability power) with Quintessences of Swiftness (movement speed)
Cooldown runes are unnecessary since you will have the Mejai's bonus along with your boots and the Crest of the Ancient Golem Buff

These masteries are set for Summoner's Rift
The Crest of the Ancient Golem Buff will be your best friend if you have trouble keeping your mana

Clarity is necessary for keeping you active during early game
Ghost can be switched with Flash, in which case you will need to take the Ghost mastery off, put it on Good Hands then put the stat on the Flash mastery and off you go
Ignite can be useful for finishing someone off without turret diving, however, her skills are long ranged and allow for easy finishing moves from safe distances

Doran's Ring is a great starting item with it's AP and Mana Regeneration, the extra HP is also beneficial if your first gank doesn't go over well
You should only get Mejai's if you know you and your partner are going to be able to kill the enemy, if not, invest in the next object and wait until mid to late game for your team to start dishing out a lot of damage
If your team is sucking and you keep dying, don't bother with Mejai's, because it's obviously useless
Rod of Ages is a good early game item so you can beef up throughout the game and stay matched with your opponents
If the enemy is building early M.res, get Void Staff early to keep up the harassment
If you can kill off enemies but are left with barely any health and have to go back, get Will of the Ancients so you can farm minions, heal up, and stay in the fight
Rylai's Crystal Scepter is good for durability if you find yourself dying, not being able to catch the enemy, can't shake the enemy off your butt
Banshee's Veil is good for protecting yourself from unfriendly mages like Veigar, who could most likely 2-3 hit you and be done with it (this is why you need someone with you, to take the hits while you protect them and dish out damage)
Mejai's can be substituted with Rylai's or Banshee's Veil
Will of the Ancients can be substituted with Rylai's or Banshee's Veil
And of course, if something is going wrong, build what's necessary and adapt to the ever changing game

Command: Attack - Short sweet and to the point, you send it somewhere and it deals damage to enemies it passes through
Command: Dissonance - Deals damage to enemies around the ball and creates a field that speeds up allies and slows enemies
Command: Protect: Used on and ally champion, it creates a shield around them, grants them 10, 15, 20, 25, 30 M.res and Armor, and deals damage to enemies it passes through (making it ideal for farming in combination with Command: Attack)
Command: Shockwave - The ball sends out energy, dealing damage and pulling nearby champions in the direction of the ball
Command: Attack and Command: Protect can be used to jungle/farm, however, all of her skills are useful for farming
Command: Shockwave can be used to keep enemies in battle or to draw them away from you
Command: Dissonance can be used while the ball is with you to give yourself and allies about to run into the field a short, yet useful movement speed boost
Command: Shockwave can also be used while the ball is with Orianna
Don't forget that the ball will return to you if you stray too far from it (this goes for if you use Command: Protect on and ally and wander too far)
If you find you and your team taking too much damage, it may be more beneficial to become more defensive, lay back, and invest a few extra points into Command: Protect

Pros

Nuke the **** out of people
Save your allies with a crazy shield
Can heal using Spell Vamp.

Cons

Squishy
Can't solo (so hope your teammates don't leave)

Please be nice to me and your fellow summoner's (my first build here)
Constructive Criticism is accepted... just don't be a douche
If you have any questions post them and I'll do my best to reply

Good luck on the Fields of Justice, Summoners, and Happy Gaming!
Thanks!