Hello Guys! So this is my first guide ever and of course I chose Orianna. I have invested a lot of time in her via game play and just additional research on her abilities and such. I personally believe Orianna is one of the strongest Mid-Laners in the game. For starters, Command: Attack (Q) can be used for amazing poke and harassment. As you level her up and buy her items her cooldown for her Q makes the perfect poke. Secondly, Command: Dissonance (W) is a great way to both escape and slow. Command: Dissonance causes The Ball to explode with energy. This energy can speed up Orianna (and Allies) or slow her enemies. Along with slowing Dissonance also causes damage. Orianna's third ability is Command: Protect (E). This ability summons The Ball toward the target, via Orianna herself or an Ally. This ability is actually so much more than just a shield. Due to its automatic return to her when summoned it can help Orianna get out of some sticky situations. For example, If Orianna were up against, lets say a Leblanc, she could use Command: Protect when Leblanc darts toward her, not only protecting her, but giving her a chance to use her other abilities and damage Leblancs play. And finally we have Command: Shockwave (R). This ability is by far my favorite to use. Not only does it cause a lot of damage but it can help set Orianna up for a great combo. You can use her Ult whenever you chose but I have found two situations where it is almost necessary.

Situation 1:
During early game when it is just you against your enemy you should not use your Ult until your opponent is roughly half health or lower. Once your enemies health gets this low you can do 1 of 2 things. You can Q and hit your enemy and set up for a wicked combo, or toss your ball out in the lane and wait for the enemy to get close. If you chose to do the combo it is important that you follow this sequence: Q>R>w>Q Not only will you do exceptional damage, but because of the abilities you used your Thunderlords will activate causing even more damage. Congratulations you have just killed your enemy and threatened the entire enemy team, making you, a complete badass. You're welcome.

Situation 2:
Late game can get a bit messy, but it is so important to use your ult in a team fight. Not only will you do a lot of damage (and get a lot of assists if your team is awesomesauce) but you will control the crowd enough for the rest of your team to make good, or even bad plays with the chance of a great outcome.

Hope you have learned and enjoyed, for the most part, this guide. Any feedback is much appreciated! Now get Commanding Summoners!

PROS

Does outstanding damage
Has a shield
Has a speed boost/slow
Can use ULT for Crowd Control
Cooldowns
Escapes

CONS

Weak early game performance
Mana Cost

Orianna is a Champion that doesn't require any specific build, though I do feel like her first item is crucial. Orianna's first item, of course, depends on her opponent.

Orianna vs AP:
If Orianna is up against an AP opponent I find it important to build and rush Athene's Unholy Grail.This item is a great starting item against AP Burst champions because of the abilities it hold:
+25 magic resistance
+75% base mana regeneration
+40 ability power
+20% cooldown reduction
along with its passive:
UNIQUE: Gain 20% of the premitigation damage dealt to champions as Blood Charges, up to 100 - 250 (based on level). Healing or shielding another ally consumes charges to heal them, up to the original effect amount.
UNIQUE: Grants 100% base health regen per 5 seconds if your current health percentage is lower than your current mana percentage. Grants 100% base mana regen per 5 seconds if your current mana percentage is lower than your current health percentage.

Orianna vs AD:
When going against an AD opponent you don't need magic resist items (unless of course the rest of the team is AP heavy) therefore, the Morellonomicon is ideal.
Orianna is getting a large about of Ability Power, A huge chunk of mana, and additional damage and cooldowns with the items 3 passives:
UNIQUE: Kills and assists restore 20% of your maximum mana
UNIQUE: Dealing magic damage to an enemy champion below 35% health inflicts Grievous Wounds icon Grievous Wounds to them for 8 seconds.
UNIQUE: +20% Cooldown Reduction

Furthering the Full AP build vs AP enemy here are some suggested items and why:
Liandry's Torment: this is an item that I prefer under both circumstances (AP&AD). This item gives you a great about of Ability Power (80) as well as 300 health. Liandry's passives are quite incredible as well,
UNIQUE – EYES OF PAIN: +15 magic penetration
UNIQUE: Dealing spell damage applies a damage-over-time effect that deals bonus magic damage equal to 2% of the target's current health per second for 3 seconds. This bonus damage is doubled against movement-impaired units and capped at 100 damage per second vs. monsters.
Two more very crucial items for Orianna's full AP build are Void Staff and Rabadon's Deathcap.
Orianna is great at sending a huge AP burst all at once (her combos). With Void Staff and Rabadon's Deathcap she is adding raw power to that burst.
The last item for the AP build is, like the others, completely situational. I, personally, like to get Ludens Echo This is an item that helps in the most simple but remarkable way. You gain 100 Ability Power and 10% movement speed, along with its passives that do extra bonus damage when this item is fully charged. You might not think that Orianna needs anymore movement speed, but when you get in some sticky situations its great to have, especially when you activate Command: Dissonance to escape.
As I have said before her build is completely up to you, these are just my suggestions and the build I typically follow.

As for going against an AD opponent my build is slightly different. I like to chose items that give good AP burst as well as a large amount of health to lean a little towards the tanky side. Those items include the following:
After building my AD starting item I chose to build Zhonya's Hourglass,
This item doesn't give off as much AP burst as other items but it does give you a large amount. Not only does Zhonya's give AP but it gives Orianna Armor and 10% Cooldown Reduction that is good to have against an AP opponent early on. If you find yourself in a sticky situation and are looking for a quick minute to get yourself together to make a hasty escape, Zhonya's Hourglass is perfect. With Zonya's passive you are able to activate it and become invincible for 2.5 seconds, while being rooted of course. This is the perfect opportunity to focus on your escape and possibly save your life.
There are no more specific items I use with my AD build, just something with more AP burth and a large amount of health, via Liandy's Torment, Rylai's Crystal Scepter, Hextext GLP-800, etc.