The word oasis conjures visions of palm trees; pure, refreshing springs; olives and dates; the heady fragrance of citrus and jasmine - scent memories of beautiful times and places; camels; and the colorful tents of nomadic tribes. Perhaps this list is based on romantic notions of distant topoi, such as the Silk Road, imagined by this Northern-dweller. Your list of associations will be different than mine, I am sure.
Oasis evokes a peripheral lexicon of words that shelter one from difficult circumstances, if only as metaphor, such as island, sanctuary, paradise, refugia, and utopia. Mirage, a related term, is not necessarily an hallucination (as the optical effects are real), but is no more useful than a dream or a vision, without the tangible benefits of an oasis.
Technically, oasis is defined as a fertile spot in an otherwise barren region. The isolated area of vegetation is fostered by the presence of a natural occurrence of water in an arid zone, either from a rare surfacing of a subterranean water source or from a geology that has the ability to retain moisture, unlike the surrounding sands.
An oasis can be real or illusory, a place that provides all that is fundamental for survival, nourishing both the physical and the spiritual.