This paper presents the design and evaluation of an oxygen low-pressure alarm for use in conjunction with medical oxygen cylinders. Patients, who are dependent on oxygen supply through portable, high pressure oxygen cylinders, rely on staff vigilance to ensure that the volume of oxygen in the cylinder does not drop below a critical level, thereby compromising their safety. To minimise such risks, a low cost, mechanically driven, easy to interface, low pressure alarm was developed to generate an audible warning intended to alert patients and their carers of depleting oxygen pressure levels. The device is designed to interface with portable medical oxygen cylinders and their pressure regulators. It implements a spring-loaded diaphragm mechanism, which opens a vent when the pressure level in the cylinder drops below a pre-set level of 15 bars (1.5 MPa). The gas then passes through a nozzle incorporating an orifice, thus generating an audible whistling sound. The proposed device is compact and cost effective in terms of manufacture and power consumption. (4 pages)