H2/O2 mixture is not flammable if concentration of oxygen is below 4% - regardless of pressure. World record of deep-sea diving - to be exact, "living in an ambient pressure habitat" - is 701 meters, and was established by French divers breathing 99.5% H2, 0.5% O2 mixture. At that depth partial pressure of O2 was 0.35 bar, well below oxygen toxicity level, and the gas was so dense that heat flow was a major problem. I don't remember off hand at what temperature the habitat was kept (close to 30 C), but I remember that the margin of error was only 1 degree C in either direction. More than 1 degree colder or warmer led to hypothermia or overheating, respectively. Considering that sea at that depth is uniform 4 C, and water carries off heat VERY quickly, death by hypothermia was a very real risk for these Frenchmen.

Anyone still up for diving Europa? ;)