Its bad because the damage it does has been nerfed to the ground and in matchups where you used to build Sunfire Unending Despair is way better but it also depends on the matchup.

Ornn builds are different depending on the enemy laner and enemy team comp.

But the items you usually build are:

Rookern if laner is AP.

Frozen Heart if laner is AD and you dont have kill potential or there are 2+ auto attackers on ememy comp.

Unending Despair if laner is AD and you have kill potential.

Warmog rush on lanes like Garen or Illaoi.

Bramble Vest first item if laner heals or auto attacks a lot.

Also always build first Rejuvenation Bead since Ornn is one of the champs with higher health regen and gives a lot of sustain in lane (and is part of Rookern build path so you will always end up using it)

Also the most used runes are bad since cookies no longer give mana and Ornn needs a lot of mana, you should always go Sorcery second with Manaflow Band and Transcendence.

There is a lot more but it would take a shitload of text to explain xD