Introduction
Hello, my name is Gustavo, but I go as Pikkachu Loiro on the Brazilian server, I'm a main Ornn player with almost 200 games played on him in this season alone, this is a very detailed guide about how to build on Ornn! Make sure to read the notes that I made on every build and rune!
The most important thing about playing Ornn is having fun and creat anger on your adversary by making sure he can't kill you!
Runes
Those are the regular runes to be safe in the lane, make sure you have or double armor or double magic resist if you know what kind of damage your adversary does!
As Ornn, all you want to do is scale and grow bigger as the game goes, grasp of the undying and overgrowth will increase your life bar substantially, demolish will make you deal more damage to towers making life easier to destroy tower platings and the golden jewel of your build resolve rune is conditioning, because all you do as Ornn is build resistances, and this gives you a 5% increase in your stats!
The secondary rune (inspiration) makes sure you don't have to spend money on boots and increase your early game sustain on the lane with biscuit delivery.
Itemization
Make sure youo build at least 2 items that you can upgrade at level 13. Sunfire Cape is a must have item as it increases your damage a lot! Agaist AD finish this item first them build abyssal mask. All of this tips are on their own items build
Spells
Flash and Teleport are very importatn for Ornn, as a low mobility champion you have to make sure that you can get out of tough situations with flash and yor E ability and go quickly to lane with the teleport.
Abilities
Ornn does not have the same sustain in the lane as he used to be, so to make sure you have a good poke in the lane is very good to max your Volcanic Rupture (Q) first. It cost almost no mana, and it is the safest ability to farm with!
Make sure to hit the Q+W+E combo to make the most damage to minions and to your adversaries.

Passive - LIVING FORGE
Ornn increases his bonus armor by 10% and bonus magic resistance by 10% from all sources.

Ornn can purchase non-consumable items from anywhere on the field by forging them himself, and is also presented a Forge Menu with his recommended items to select to be forged. Forging takes 4 seconds and the process is interrupted if Ornn enters combat, disabling Living Forge.

When Ornn reaches level 13, any upgradable (MASTERWORK) item Ornn owns or purchases for the rest of the game is upgraded for free, up to a maximum of two. Additionally, for each level after 13 (level 14, 15, 16 and 17), Ornn can upgrade one of his ally's upgradable (MASTERWORK) items by getting within 600 range of them and clicking on them, up to a maximum of 1 per ally.

MASTERWORK items: Might of the Ruined King, Molten Edge, Obsidian Cleaver, Trinity Fusion, Youmuu's Wraithblade, Luden's Pulse, Rabadon's Deathcrown, Zhonya's Paradox, Forgefire Cape, Frozen Fist, Infernal Mask, Circlet of the Iron Solari and Salvation.

Ornn's basic attacks against Brittle enemies briefly knocks them back.

Enemies made Brittle by Ornn take 10−18% of their maximum health as bonus magic damage the next time they are immobilized by Ornn or his allies, consuming all Brittle stacks (depending on Ornn's level).

VOLCANIC RUPTURE
Ornn slams his hammer into the ground, sending a fissure in the target direction that stops early if an enemy champion is hit, dealing 20 / 45 / 70 / 95 / 120 (+ 1.1 per total attack damage) physical damage and slowing enemies hit by 40% for 2 seconds.

After a 1.5 second delay, an impassable magma pillar erupts at maximum range for 4 seconds.

BELLOWS BREATH
Ornn becomes unstoppable and marches in the target direction with 35% reduced movement speed for 0.75 seconds.

Over the march he belches fire in a cone in front of him, dealing 12 / 13 / 14 / 15 / 16% of target's maximum health over the duration to enemies in the AoE (80 / 130 / 180 / 230 / 280 damage minimum, which is also the damage dealt to minions). Monsters take 155 / 180 / 205 / 230 / 255 damage over the full duration of the march. The final bout of flame will apply Brittle.

SEARING CHARGE
Ornn charges forward, dealing 80 / 125 / 170 / 215 / 260 (+ 40% bonus armor) (+ 40% bonus magic resistance) physical damage to enemies he passes through. Upon colliding with terrain, the dash stops and creates a shockwave, knocking up nearby enemies for 1 second.

Magma pillars and terrain created by enemy champions caught in the shockwave are destroyed.

CALL OF THE FORGE GOD
First Cast: Ornn summons an elemental at maximum range in the target direction that stampedes toward him, dealing 125 / 175 / 225 (+20% of ability power) magic damage and slowing enemies hit by up to 40 / 60 / 80% for 2 seconds based on distanced traveled (minimum 10% slow).

While stampeding, Ornn can re-activate Call of the Forge God.

Second Cast: Ornn dashes forward in the target direction. If Ornn collides with the elemental, he headbutts it, sending it stampeding again in the new direction and empowering it to knock up enemies hit for 1 second, reduced to 0.5 seconds after the first enemy champion hit. Call of the Forge God can affect enemies twice.