Answer:
An oasis can be formed by an underground aquifer or river that creates enough pressure for water to seep to the surface, forming the oasis. These aquifers and natural springs allow for life to exist in harsh climates like the desert and are often well known to local herders, farmers, and travelers in the region.
It would be the "Western Ghats" that prevents moisture from the summer monsoon winds from reaching the interior of the subcontinent, since its positioning is such that any excess winds are diverted.
