I was always led to believe that using welding oxygen was bloody dangerous because the water content was too high for safety.
That's disinformation (one of many in GA). Best to phone up British Oxygen - I have. All o2 comes from the same plant and is made to the same spec; the difference is in the paperwork which contains a tighter spec for medical/aviation grades. Welding o2 has to be very clean because even very tiny impurities visibly contaminate the weld. Lots of people have looked into this too - there is an article on avweb.com which says the same thing.
The water content specification is tighter for aviation oxygen because you could be potentially flying at levels where the temperatures are low enough within the airframe to freeze humidity in the regulators or the pipework. In the GA context, where the o2 bottle (of a fitted system) is in the boot and the pipework is within the passenger space even if behind trims, you would be freezing your b*lls off if it got that cold. If using a portable o2 kit it would need to be even colder because the system is right next to you. I can see this originates from old military requirements where there were indeed unpressurised (and poorly heated) jets with operating ceilings high enough to reach say -50C OAT, but this is hardly GA unless you are flying to the N Pole and you are wrapped up like an Eskimo...
Loads of people fly with welding oxygen. I've been using it for years, and because it's so hard to get refills I rent the big BOC bottle and - with appropriate safety measures - I refill the bottles myself.