Hello, I am here to be your guide to Ornn.
It is important for you to know a few things. Ornn's power heavily relies on your choices during gameplay. Living Forgeallows you to build items while in lane and if you build the wrong items you will be in deep trouble. Also Ornn lacks being able to deal sufficient damage from his base attacks and must constantly use his abilities to keep up.
Playing Ornn will require you to plan ahead and properly choose what to build in response. If your opponent's team is mainly AD then build accordingly, and the same goes for AP. One of the greater challenges is deciding what order to build even if you know what you'll need later on. This guide will, hopefully, assist you in making the right choices.

No matter who plays against you in lane you should always start with Refillable Potion and Sapphire Crystal. Refillable Potion is best because you will be unable to make Health Potion while in lane and going back to get more will waste gold that would be spent on other items. The Crystal will provide you with some extra mana for early farming and trades, also being a key stepping stone into Catalyst of Aeons.
Your first items bought should be Ruby Crystal. After this point is where you must choose either to build Bami's Cinder (best if against AD champ) or Catalyst (better against AP.) Catalyst can also be useful early on due to in being able to heal and regain lost mana.
However, if up against an aggressive AP champ, like Heimerdinger then instead focus on Null-Magic Mantle.
Abyssal Mask is often one of the first items to be built, followed by Sunfire Aegis and then Frozen Heart. Boots should be purchased and built into either Ninja Tabi or Mercury's Treads.
If teamfights are impossible to avoid then build Gargoyle Stoneplate to survive while being surrounded. Activating it will cost you damage, but if you apply brittle to enough enemies then your team should be able to finish them off.
Spirit Visage and Adaptive Helm are very similar and if up against an aggressive AP one of which should be chosen. If opposing team is mostly AD go with Visage for the rapid healing, but Helm will be useful against magic abilities being spammed constantly on you.
Thornmail is a must if you are unfortunate enough to face off against champions like Darius or Vayne. High damage champions that are difficult to fight without a team.

Finally, when you get to the point of upgrading your items the two most recommended are Forgefire Cape and Infernal Mask. Cape is better for armor while Mask has magic resist.

Living Forge Allows you build items in lane, not including wards or consumables. At level 13 allows you and your team to upgrade certain items in Masterwork Items.
Volcanic Rupture Sends a fissure in the target direction and after a short time creates a pillar.
Bellows Breath Marches in target direction while breathing flames that apply Brittle.
Searing Charge Dashes forward, dealing damage to all in the way. If Ornn hits a wall causes a knock-up to nearby enemy champions and minions.
Call of the Forge God First Cast: Ornn summons an elemental at maximum range in the target direction that stampedes toward him. This applies damage, a slow, and brittle to all that are hit.
Second Cast: Ornn dashes forward in the target direction. If Ornn collides with the elemental he sends it in the new direction and empowering it to knock up enemies hit.
Teleport will allow you an quick way to return to lane or travel across the map to compensate for your sluggish speed.
Flash is always nice to have.

Ornn is one of the greatest late game champs due to him being able to turn almost any team fight around.
Use Bellows Breath to apply brittle to as many enemy champions as possible. Call of the Forge God to slow and knock-up the other team to get your side a killing streak.

Pros: New items. Great in team fights. Build items in lane.

Cons: Slow start. Easy to make a mistake. Next to no damge without items.

Ornn is still a new champion at this time, but is a lot of fun to play. Always watch for enemy ganks and don't be too aggressive. For most of early game it will much easier to stop your opponent's progress in lane than make progress yourself.