A gas supply apparatus (1) particularly for supplying oxygen comprises a gas separator (2) for separating oxygen from the other constituents of the air. The oxygen passes to a piston cavity (6) of a compressor unit (7) where it is compressed and passed to a storage means (12) from where it can be dispensed via a pressure regulator (13) to a number of emergency oxygen masks (14). The compressor unit (7) includes a number of moving parts lubricated by oil within a first cavity (8). Cavity (8) is separated from the piston cavity (6) by a purge cavity (9). In the event of an oil leak from the first cavity (8), the oil will gather in the purge cavity (9) where it is purged therefrom by the waste gas from the separator unit (2) after the oxygen has been removed. If any of the waste gas leaks from the purge cavity (9) into the piston cavity (6) it will mix with the oxygen compressed therein and form a safe mixture.