Hi, in my opinion there is no one good mythical item.
You can view it from several perspectives, but remember that I may be wrong. How many players, so many opinions:
- if there are many ranged characters, select Iceborn Gauntlet; if not, choose Sunfire Aegis because you will deal a lot of damage.
- if they are fast choose Iceborn Gauntlet; if not, choose Sunfire Aegis.
- if you are backwards, choose Iceborn Gauntlet because of the low price; if not, you can afford Sunfire Aegis.

In general, Sunfire Aegis is more likely to be chosen at low level because you are doing a lot of damage and your opponents are not playing very well. However, at a higher level, you will almost always choose Iceborn Gauntlet because you want to help your team as much as possible in the fight as a tank.

In my opinion, a good mythic for Ornn Attack Damage would be Goredrinker, because the mythic passive provides Cooldown reduction, which greatly boosts up Ornn's outplay potential. Don't really have any other ideas for mythics, though.

Prowler's Claw I haven't tested, so I won't say.
I used to try Stridebreaker and in early it was nice to play. Unfortunately, the late-game was poor. I explain why: your task is to play teamfight well and absorb as much damage as possible. Also on the sideline for split push, your job is to survive and use your Teleport for team fights. Unfortunately, this item mainly provides AD damage and speed attack instead of armor and magic resist. In ranked games, it is better to build basic items.

In my opinion Anathema's Chains is very good when enemy have one feed hypercarry. Otherwise, build: Randuin's Omen, Frozen Heart or Force of Nature . But yes, the proposed item is just great, build it!

Unsealed Spellbook will be added in the next publication.
Manaflow Band and Transcendence, solves mana problems and provides CDRs that Ornn likes very much.
Iceborn Gauntlet is also considered even at the highest level. Don't underestimate this mythical item. Provides health that scales with passive Living Forge.
Bone Plating can be effective in a variety of situations, and so is Second Wind or possibly Conditioning.
Aftershock is an optional, but effective choice. Better choice is Grasp of the Undying for the most part.
After all, I have described why Spirit Visage is a very rare choice and why Warmog's Armor is a valuable item in my opinion.
Why should I upgrade second E? It has a very long cooldown and doesn't help with farming as much as Q or possibly W.
Mordekaiser is a very tough opponent. He has a lot of tools that Ornn destroys. You can see it in all the statistics and I know it from my own experience.
Of course, Fiora can use W when Ornn uses Q. I don't deny that anywhere. Fiora has a long W cooldown. I've only mentioned when Fiora can stun Ornn.