@freespeed2, your comment regarding marginal O2 partial pressure at FL380 is entirely correct. I think the official limit is FL400 for positive pressure breathing apparatus requirements. However, the TUC comment isn't: the 12-15 seconds at FL380 is not for an "acclimatised" body. If it was acclimatised, decompression wouldn't be a problem... get it? ;-)
It rather depends on what it is you're doing (physical activity), and on your physical predisposition. Oddly enough whether you're fat or lean makes no difference, there are other factors at play, many not well known to aviation/mountaineering medicine.
The point is, 15 seconds is a very short time if you're caught by surprise, and there's no time to debug a problem if there is one, e.g. an insufficiently opened O2 bottle valve (not that pilots could do anything about that anyway), or flow regulators in the wrong position etc. You simply don't get a second chance.