Orianna is excellent mid.. start with boots, two hp pots, and a mana pot. This will let you stay in mid long enough to harass and outlast your opponent. Go base when you reach level 6 or when your opponent dies c: You want to get your boots of Lucidity first to help with harass and the move speed to avoid damage and escape ganks. Avoid your Protect at all cost unless it is COMPLETELY needed. You need all the damage spells first to maximize harass so your enemy can't farm and you push them to go Base. Next, you need to grab a mana gem to stay in lane longer. At this point you are still vulnerable so I usually grab HP pots and a few Mana pots just to survive longer until my teleport is up.
Remember, you don't necessarily need to use teleport when you go B. If your enemy isn't a threat to the tower or you can't gank them with wards then jsut run back and save your Teleport for a more needed situation.
After get enough gold, I grab a Catalyst and grab a Blast Wand if I can. Build your RoA FAST. After RoA, grab either a Mana gem or a Meki Pendant for the Regen. Get either one, but after you get enough gold, build a tear and LEAVE IT ALONE! Your bonus will take about 10-15 minutes to max out. So, next you want to get either a Blasting Wand or a Giant's Belt. You always want to get what you need for the situation at hand. Build your Rylai's and help with ganks. By this time you should have earned enough gold to get a Needlessly large rod, Get it.. Build your Rabadon's and then your tear should be maxed out or very close to it. so grab a blasting wand and build Archangel's Staff. By this time it should be near end game and you need to build for more HP and mana for pushing and outlasting the enemy team. For this, you need to build another Catalyst and the build it into a Banshee's Veil, this will help with MResist and blocking an ability for a hasty escape. ^_^ Thanks for reading this and thumbs up for more builds. I'm open to critizism so feel free to comment with ideas and alternative.