Sufficient oxygen is required to meet the patient's requirements. It may also be used to provide a background rate of gas flow (bias flow) through the breathing circuit, or to control the ventilator cycle itself.

During critical care transportation, oxygen is commonly supplied from gas cylinders. Ambulances carry a pair of code F (1360 litre) or HX (2300 litre) cylinders in the cabin and two small D (340 litre) or CD cylinders (460 litre). Modern CD and HX cylinders have similar dimensions to their counterparts but are filled to 23 000 kPa rather than 13 700 kPa. They include integral four bar pressure regulators and Schraeder outlets.

Ventilators incorporating gas compressors (e.g. LTV-1000) can also utilize various low-pressure oxygen sources to enrich ambient air. Oxygen concentrators can be used if supplying cylinders is problematic, such as in a military field hospital. For aeromedical evacuation, low-pressure liquid oxygen systems can provide three times more gaseous oxygen than a similar sized cylinder.