I just am not feeling Preordain specifically in Oath. There's not an Oath curve that I see Preordain being a great fit in.
Oath is defined by its marquee two-drop and really appreciates off-color mana.
Preordain is really best utilized by decks with low curve that will often have a single spare on-color mana on some of the game's critical turns. Either to cast the Preordain, or more importantly to be able to cast the Swords or Pyroblast or second Preordain that you Preordain into.
In Oath I just see Preordain adding a lot of air. I can't see the upside to Preordaining with your Forbidden Orchard into X with a Mox Pearl open, and no second land in hand. You just waste an entire turn then I think?
I think any deck that isn't a Xerox variant or an Ancient Tomb variant (where it is easy to use up all your mana every turn) really needs to understand how it will curve out in both Mox and non-Mox situations. Given that the current best decks have fractional mana curves, I would really like to take Oath in a different direction so as to not just become a worse version of something else. Just jam threats, they don't have to be super expensive but they probably should cost more than half a mana.
I dunno. Maybe I'm crazy. I'm turning my attention to Oath soon and will think through this some more.
It's about finding the cards you need man. If that doesn't make sense in an Oath deck, where does it ever make sense? In particular the strength of Preordain over the oft-considered more-powerful Ponder expresses itself most coherently in this strategy. Just like Dig Through Time can put valuable cards on the bottom of your library, protecting them from being oath'ed away and giving you a crucial few turns after a nasty amount of self-milling, Preordain puts two cards on bottom quite often with it's scry ability. In contrast, Ponder may leave you needing to shuffle, which can be detrimental. I always play one Brainstorm, and either three or four Preordains, but sometimes one Ponder in place of a single preordain. The shuffling can be nice with Jace, the Mind Sculptor, and that's the only real reason I would prefer it in an Oath deck.