When Ned Beatty rushes in to put the girl on oxygen during her coughing episode, he opens the valve that controls flow from the tank to the regulator. Once that valve is open even a bit, the full pressure from the tank is then at the regulator. He keeps going back to that valve and turning it. That would do nothing to regulate the flow of oxygen to the patient. He never touches the regulator knob, which is the only way to adjust the amount of oxygen getting to the patient. Oxygen flow was not seen in the humidifier (the bottle of water at the end of the regulator). The oxygen tank was probably an empty prop tank. And the sound of oxygen flowing was probably a sound effect.