Yes. The stopper holds the oxygen concentrated air in the reservoir when filling it prior to use. Due to the design parameters of the reservoir, air will flow out much faster than your concentrator is able to fill if the cork is not in place to stop the oxygen from exiting the reservoir. The first place to look if your reservoir is not filling fully is to check the cork stopper. Additionally, when not in use or during transport, we recommend using the stopper cork to prevent unknown items or dust from entering the reservoir.