Answer:
WELL...
Firstly, an oasis is a small patch of vegetation (Plantlife) surrounded by a desert. Many communities around the area plant strong trees such as palms around outside or perimeter of the oases to keep the desert sand away from the crops and water. Oases are made by a source of a type of fresh water in a dry or arid reigion.
Now, why does it occur? I don't know. Ha... But I do know that it's really beneficial to the community living in that area. But I'll tell you how they're formed. An oasis can be formed by an aquifer or river that creates enough pressure for water to flow past or seep to the surface, forming the oasis. Kind of like a volcano eruption except is a lot more gentle and water instead of magma or lava. These aquifers and springs allow for life to exist in harsh dry climates like the desert...etc.
Hope I helped!!!
GL :)