Before any criticism, I just want to say that I am doing this just for fun and it is my first time making a guide. This may be considered a noob build or guide when it comes to ranked matches but let me just tell you, a maxed out cool down Oriana with sufficient mana can literally play ball with her opponent and it is TONS OF FUN. You will be moving your ball every chance you could get with your Q having 2.4 sec cool down mixing it with your E and W.
So why Cool Down?
As most of you might have noticed, Oriana's AP ratio isn't exactly high, so I think building her purely AP is downright a waste of her potentials.
I would say that for runes, it really depends on your play style but since we are going with a Cool down build then:
9 glyph of the focus: so that it will complete the 40% max cool down together with the evil tome and boots of lucidity.
The runes of Insight help you get that extra damage that Oriana seems to lack, or does she now? ;). However for the Quintessence, you have a choice to replace them between movement speed or health runes, personally I go with either movement speed or spell penetration.
Finally, for the seals, its anybody's choice. I go with the mana regen since it saves you from going B early game and helps you stay in lane, given that you are using your ball properly and you aren't getting damaged.
Masteries And Summoner Spells
Mastery nothing new here, i use the standard 9 - 0 - 21 caster build.
As for Summoner Spells I wouldn't restrict anyone to any certain summoner spells, if you feel comfortable with it then go ahead.
I may be wrong but, there are so many number of ways you can build Orianna, hell I wont even doubt an AD Orianna build. But for now, we stay focused to this guide's build, and that is Cool down.
For some of you that don't know, the max cool down a character can achieve is 40%. With that in mind you can switch around certain items for different alternates. Like say, if you are confident in your skills, and think you can get a full 20 stack from that Meija, then that 15% cooldown reduction gives you space to replace your Lucidity boots with a sorcerrer's boot allowing for more damage.
In Any case, your core build should be something like this:
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