Running with:
omnilib 1.0.4
omnimatter 1.2.4
omnimatter_crystal 0.1.6
omnimatter_science 0.2.8
Without Bobs, Angels or other modpacks that add new ores.
Iron Crystals are required to make Omnipacks, but the recipe is not unlocked by any technology.
Using "What is is really used for" with SHOW_ALL=true says this for Iron Crystals: "Locked behind: Technology not found".
The Crystallology 1 tech, which I think might be meant to unlock this, only unlocks the three Omnine recipes and the Omnine shards recipe.
The same is true for Iron-Omnine solution.
Iron ore salting is available, locked behind Crystallology 2.
I can manually unlock the recipes with:
/c game.player.force.recipes["iron-ore-crystalization"].enabled = true
/c game.player.force.recipes["iron-ore-solvation"].enabled = true

There also seems to be a bit of a problem with the order of the technologies, as according to What is it really used for:
- Iron Crystals require Iron-Omnide solution
- Iron-Omnide solution requires Iron Omnidic salt
- Iron Omnidic salt is unlocked by Crystallology 2
- Crystallology 2 requires Omnipacks
- Omnipacks require Iron Crystals

Am I missing something obvious here?

I've 'fixed' this by changing the end of data-updates.lua from:
omni.lib.add_unlock_recipe("crystallology-"..tier+1, ore.."-salting")

to:
omni.lib.add_unlock_recipe("crystallology-"..tier, ore.."-salting")
omni.lib.add_unlock_recipe("crystallology-"..tier, ore.."-solvation")
omni.lib.add_unlock_recipe("crystallology-"..tier, ore.."-crystalization")

This has made all the recipes available, and made Iron Crystals available at Crystallology 1, so they can be used to make the science packs to unlock Crystallology 2.