You've just been thrown out of the airlock with out a spacesuit. However, you do happen to be wearing a SCUBA style breathing apparatus. How long would the vacuum take to kill you, even though you have access to oxygen?
A SCUBA style breathing apparatus wouldn't significantly extend your survival time in a vacuum. In fact it might make your already fatal situation worse.
Without access to oxygen your timeline of usable consciousness in a vacuum is 6-9 seconds with death following soon after.
Since the only thing that would be pressurized would be your respiratory system. their is a significant pressure differential between your insides and the vacuum of space that is only contained by your fleshy body. We call damage caused by such pressure differentials Barotrauma. In addition to that it's highly plausible that that same differential would be enough to blow the mouthpiece out of your mouth.
I suspect the SCUBA mask would be insufficient to keep you from dying from asphyxiation.
While a SCUBA mask may be waterproof, the vacuum of space is a slightly different degree of pressure differential, and one for which your mask isn't designed. I think the mask's seal would be incapable of preventing the air from escaping into space.
I believe you'd be frozen solid before you could think about breathing.
looks like it would take a while, or forever, to freeze.
The pressure differential would make it impossible to breathe. Upon being released into space if you don't exhale the loss of pressure would cause your lungs to burst. Any air in that tank, supposing you can actually suck air out of it, wouldn't make it to your lungs as it'd go right out of your mouth into space.
Also I don't think you could keep oxygen in your blood at that pressure, and you'd die of hypoxia.
So long story short, you die in seconds, with or without a scuba apparatus. In fact, the only use I can think of for a scuba apparatus in space would be to chuck it in the opposite direction of your ship, to slow or reverse your velocity away from said ship (not that it would help much, but still).