The oxygen is a red herring.
Most likely he had gut rot - food poisoning - which may cause colicy pain ie pain that comes and goes. It got better. Again coincidence
The main issue with depressurisation is if there is gas in a body cavity. If the guts are not working they absorb gas. Depressurisation causes the gas to expand, causing pain and possibly rupture of the guts. If the guts are handled, they stop working for a few deays (ileus) so flying after an abdominal operation is a no no and we normally stop passengers flying for two weeks
Tumours and other pathology may also cause obstruction so that the guts upstream from the obstruction become distended - same problem
A laparoscopy - keyhole surgery - passes gas into the peritoneum - the abdominal cavity. This too will expand on depresurisation so flying is normally banned for 3 days
Gas in other cavities - the chest, joints and teeth - can also cause pain on depressurisation
Flying freight doesnt have this problem - unless it is livestock