I love playing the "Demi God" its the champion i have highest %win on around 65%. In General ill explain basic stuff such as combos and Living Forge (his passive).

COMBOS

The main Combo with Ornn is Q then E the pillar. While the enemy is knocked up use W then auto. This will not kill the enemy (ofc it does if your target is low) but give you a nice poke. If you see a opportunity to kill go all in. Since you gain sheild with Bellows Breath dont be too afried but you're not invincible. Try Q-W-E Searing Charge the pillar to finish Brittle
Then there is one harder combo this is more like a kill move: First Q-R-W-Auto-Ultrecast-E(on pillar) and finish with a auto.

THE LIVING FORGE

Most games you win lane because you really never have to reacall. Forge Sapphire Crystal first no matter what you buy first. This is why you dont run out of Mana unless your spaming you spells. But dont be afried to farm with Volcanic Rupture. When you hit lvl 13 Masterwork is available for u and your teammates.

Getting ganked?

When youre getting ganked recommnd going into the bushes or E away. If you think can 2 v 1 em, head to the bush. There try bait out 1v1. If not possible ult inside the bush so they cant be sure were is going. Then all in one of them. Since they gonna change the W shiled i would not recommend this anymore. Beacuase the W shild i the main reason this works.

While ganking with Teleport
start with ult or Q i recomend ult. When enemy is stund W and leave your adc to do the rest.

BE Careful

Playing Ornn can be sometimes be hard since you have to set up your gap closer with Q. Lets say you face [Vayne] or [Lucian] then your good as dead. Just stay under tower and dont feed.

This guide is work in progress

Tbh almost every game you build same things. Not to much to switch up on.

My favorite Item on Ornn is Iceborn Gauntlet. Second place goes to Warmog's Armorand third place goes to Abyssal Mask. I my opinion is Iceborn Gauntlet the perfect item on Ornn. It gives you a good amount of armor and Mana. It also gives 20% cdr which SUPER good and still has a great Passive

All tryhard games on Ornn you always build tank. You acually get really tanky after 2 items. Usally after your Warmog's Armor. Going Warmog's Armor when you dosent have over 3000 hp. (to trigger WARMOG'S HEART)

Your job is pretty straight forward. STUNDS,STUNDS,SLOWS. And of course some DMG
If youre team has immobilizing abilities such as Dark Binding or Flawless Duet you can focus more on making enemys Brittle with you Bellows Breath

If YOU are the one to start the fight, i like to open with Call of the Forge God then Q-E-W-auto. They should be dead by now.
If you somehow miss your recast on Call of the Forge God. The fight might be lost tbh.

But you gotta try right!

Do your best to land Volcanic Rupture and stun em with Searing Charge more then that you cant do.