According to the common natures of oases in the world, scientific conception of oasis is given out. Based on a lot of the previous research results, the distribution regions of oasis, the objective conditions forming oasis, the hydrological environment of oasis, oasis vegetation types, oasis soil types and other geographical features of oasis are induced systematically. In the same time, the cold island structure of oasis, the cold island effect of oasis, the humid island structure of oasis, the humidity inverse of atmospheric surface layer over desert near oasis, surface high utilizable energy (or net radiation), wind barrier effect, dynamic effects, thermodynamic effects, effect of enhancing rain and other climatic effects are summarized comprehensively. And from the geographic features and climatic effects of oases in the world, the essential properties, such as systematization, dependence on water, high contrast, proper scale extent, high efficiency, frailty and gradual evolution, which is of all oases, are pointed out. Finally, the effects of human action on oasis are discussed, and the problems and suggestions on oasis research are inquired into.