Oases can be a natural phenomenon that is formed naturally by nature. But they can also be man made and their sources may come from several places. There is also the possibility that an oasis can be formed as a result of an underground aquifer or a river that creates sufficient pressure that allows for water to seep to the top, which ultimately forms the Oasis.
In deserts, the underground water source can come up due to several reasons. One of these reasons can be that water below gets accumulated in a certain area that has lots of rock formations below. Since the water cannot percolate and seep further into the ground, the water comes up as it piles up and the soil above becomes fertile for the plants to thrive.