Abstract: An oxygen generating apparatus according to the present invention includes a reaction vessel and a cartridge. The cartridge is constructed for insertion into the reaction vessel, and includes a cartridge plate and a plurality of reagent tubes holding oxygen-producing reagents. The reagent tubes, which include at least one short tube and a plurality of standard tubes, each have an upper end coupled to the cartridge plate and a lower end which has an opening or port. When the cartridge is inserted into the reaction vessel, each of the plurality of standard tubes extends substantially to a floor of the reaction vessel, while the at least one short tube extends to a point remote from the floor of the reaction vessel. The cartridge may include an activation plate which causes the release of the reagents into the reaction vessel by pulling up a retaining sleeve when the cartridge is inserted into the reaction vessel.