In general, you should not settle on an oasis. Settling removes the oasis feature from the tile, meaning you just settled on a dry desert tile, yielding 2 food and 1 hammer, plus the palace yield and anything else you build in the city center (such as granary, etc.). You will get the normal Petra yield from the city center, but because the oasis is removed, you don't get the Bazaar gold bonus for the oasis (since the oasis is no more) and adjacent tiles do not have fresh water access (since they are no longer adjacent to an oasis), which means their yields (even with Petra) are bleh. Far better to settle next to the oasis.