With recent changes, many trials and errors I have produced build that kind of maximizes the usage of his passive and is relatively not that expensive gold-wise. Just tank yourself into extreme values.

Ornn is mainly a Tank, but has qualities in his kit that let him whip out high damage in simple combos. Not being only a meatshield, but also self-dependent damage dealer is pretty cool right, but did you ever expect to be able to CC-lock people through the entire time they are getting beat? Here you go. Are you still unsatisfied? Fine. Ornn has ability to upgrade every Mythic Item in the game, granting him and his teammates extra stats. That should be sufficient, no?

Ah, I completely forgot. Ornn has also other important passive. Starting with 10% more bonus armor, magic resistance and health points, then getting +4% for every item upgraded. (First upgrade at lvl 13, then one on every level-up till lvl 17) This puts us with +30% tank stats in the lategame.

And we are not done yet folks. Ornn can use shop anywhere on the map BUT cannot purchase any CONSUMABLE items. He gets full effect of armor/mr from the items, but due to nerf in patch 10.13 you only get the % of health and mana that you are currently on.

Q -Volcanic Rupture- straightforward AOE skillshot that deals damage, slows and creates terrain. Q erects pillar-wall at the end of its range or right after hitting enemy champion. Mana cost of this ability doesnt change with leveling up, so thats the main reason why I would recommend maxing it first. Scales strongly with AD.

W -Bellows Breath- AOE cone of fire. While using it, Ornn will step forward a little, also being "UNSTOPPABLE" through the entire duration. (If hostile CC ability would last longer than W itself, it WILL RETAIN its effect.) If you manage to hit enemy with last proc of W, they will be BRITTLED - they take hp% as bonus magic damage the next time they are autoattacked by Ornn, or generally immobilized by Ornn or his allies, consuming Brittle status. W doest scale with any value, only with its level.

E -Searing Charge- quite long dash that ends with a knockup when hitting terrain. AOE damage scaling with armor and resistance. Destroys any champion-made terrain, ex. Azir's R, Taliyah's R, Anivia's W etc... BUT doesn't break YORICK'S WALL (as it is coded as a "minion")

R -Call Of The Forge God- Ornn summons projectile that he can hit with R2 and knock up hit enemies. R applies BRITTLE on both casts. Scales very slightly with AP (don't). Gets blocked with abilites of champions with threat scale lvl 5 in this guide.

Brittle is the powerhouse of the Ornn. Main damage source really.