The Orbiting Arid Subsurface and Ice Sheet Sounder (OASIS) mission concept is the first to directly explore the signatures of climate change beneath the surface of two of the least well-understood arid regions of the Earth: the polar ice sheets and the hyper-arid deserts (Figure 1). With these first-of-a-kind measurements of land ice and shallow aquifers, OASIS has two well-defined science objectives. The first is to determine the thickness, inner structure, and basal boundary conditions of Earth's ice sheets to understand their dynamics and to improve models of current and future ice sheet response to climate change and, hence, to better constrain ice sheet contribution to sea level rise. The second objective is to perform detailed mapping of the spatial distribution of shallow (<100 m deep) aquifers in the most arid regions on Earth to understand groundwater hydrology, enhance groundwater flow models, and provide new insights into available water resources and paleoclimatic conditions. These two mission objectives, which align closely with two NASA Earth Science program objectives on climate and water cycle, are achieved using measurements made by a single, low-cost and proven-heritage instrument: a 45 MHz center frequency radar sounder with 10 MHz bandwidth. The OASIS radar is similar to instruments on two successful Mars missions, Mars Express and Mars Reconnaissance Orbiter, presently probing the Martian subsurface.