An air separation unit for an OBOGS includes a housing having an inlet for receiving a wet inlet air and an outlet for outputting a dry product gas. The housing includes an outer side wall and annular walls defining a series of concentric annular chambers. A first annular chamber is coupled to the inlet and includes a desiccant material to receive the wet inlet air and output a dried air. An unfilled second annular chamber is coupled to the first annular chamber. A third annular chamber is coupled to the second annular chamber at a first end and the outlet at a second end. The third annular chamber receives air separation material to selectively remove unwanted constituents from the dried air and output the dry product gas. A tap may be coupled to the second annular chamber so that dried air may be removed from the housing.