to conquer an oasis, you need hero mansion lvl 10 ($$$) and enough troops to take out the animals there. run the combat simulator against them with your force and see how you'd make out.
on one hand, they give you the bonus of whatever, +25% something, AND the animals are constantly multiplying (slowly), so you want to get it asap.
but on the other hand, getting hero mansion to level 10 is fucking crazy expensive for some reason, and you also need a sizable enough amount of troops to do it.