Let's make this clear, On-hit orianna isn't exactly the most legitimate build. I've never tried it in a ranked game, although I've done well in normal games. If you are expecting a build that can help carry your ranked team to 2000 elo click on one of the guides at the top right hand corner of this page. This is only a trolling build, although unlike other troll builds, you can expect to actually do a descent amount of damage. Don't get me wrong, you aren't exactly teemo here, but there's nothing cooler than owning a noob gangplank as on-hit orianna. Let's get to it then.

Pros

Capable of doing damage!
A unique way to play a unique champion!
Fun to play!
Troll your team and then show them you're
capable of something!
Can support your team as well!

Cons
Hard champion to master :(
Only effective if fed :(
Orianna picks her nose when you arent looking :( (jk dont hurt me)

-Remember that in early game you are still capable of doing damage with your spells!
-Your Command Ball:Attack spell can be used to check bushes and other important areas! (getting steamy in here or is it just me? Just me? Ok.
-Of course you wont do well if your a damn cowboy! In other words, don't run in before the tank!
-Dont tell your team that you are being on-hit orianna for more troll effectiveness in game!
-Make sure no one on your team sits there and picks their nose in a teamfight. They need to be presentable! Gawsh.
-Have fun. Or try to. Your team may not :)