Desert : Oasis :: Island : ? — Solving the Analogy

The answer is continent.

Reasoning:

• A desert is a vast, barren area; an oasis is a small, fertile spot within it.
• An island is a relatively small landmass surrounded by water; a continent is a vast, primary landmass.
• The relationship is part-to-whole (microcosm within macrocosm) or minor feature within major biome.

Thus: Desert is to oasis as island is to continent.