Slitherine

Playing as Ptolemy, I conquered a desert region with a Settled Oasis because it had Olive Oil. Cool! i thought. "I can stop buying Olive Oil from Carthage." But after the conquest, the Settled oasis disappeared. ??? And i didn't see it in the list of structures that could be built. Was it destroyed in the fighting? or am i not allowed to have a Settled Oasis?

I think a settled oasis is an Arab thing, it has special rules and restrictions.

Go to the campaign selection screen and pick araby, it mentioned it in there.

Only factions with the Settled Oasis modifier can have those. This includes Arabia and most desertic tribes.