You cannot claim an omphalos in an area with an enemy unit. Can't remember where in the book that is, but im pretty sure its under the absorb description
Thats not correct.
"A divitiny can absorb 1 omphalos of their choice from their area, as long as the omphalos is either unclaimed, or claimed by a friendly unit."
No enemy unit can claim an omphalos that is in this area"