Drowning is from a medical standpoint more complex than the simple notion I grew up with which was in essence "water fills your lungs so you can't breathe air".

Drowning is from a medical standpoint more complex than the simple notion I grew up with which was in essence "water fills your lungs so you can't breathe air". In fact drowning does not require filling the lungs completely. Even a volume of a few milliliters/Kilogram of body weight is enough to cause drowning. Additionally, drowning can cause serious damage to the lungs themselves even if the patient survives initial attempts at resuscitation. The alveoli (functional unit of the lungs) are lined with a surfactant that is critical to the exchange of air to the blood stream. Water can severely disrupt the surfactant and impair function not just while the water is present but until the body is able to restore the surfactant layer. Damage to the patient's lungs in this case seems to have been mild enough that the oxygen therapy could do it's job. Also notable is the 5 degree celsius water temperature (41 degrees Fahrenheit). This water temperature compared with the temperature of an olympic practice pool (~76 degrees Farenheit) is cool enough (though not as cold as many other reports) to trigger the so called "diving reflex" where stimulation of thermo-receptors in the skin triggers a vagal response that shunts blood away from the periphery and to vital organs. Minimal surfactant damage and the diving reflex (as well as the patient's age) seem likely to some degree to have facilitated successful treatment of the patient.