ALL dives are compulsory decompression dives.
We must decompress. We do get out of the water, obviously.
You begin to decompress as soon as you rise above the equilibrium point of the absorbed inert gas for the fastest tissue in your body, and you continue to do so until you are fully desaturated, which is, from a statistical point of view, at ten half-times of the slowest tissue group. Most computers consider the slowest tissue group to have a 10 hour time, so that puts the "desat" time for most computers and dive planning tools at 100 hours.
The entire concept that the "major agencies" push about "no-deco" diving is a farce and IMHO is one of the worst "misfeatures" of how they teach this sport. If you think about how they teach you to dive, and the tables and such they use, its obvious they're being fast and loose with terminology, but few people analyze it at that level.
That's too bad, because understanding this is CRITICAL when things go wrong and you are trying to figure out what you want to do about it.
We call dives "deco dives" if the CEILING is less than 0 fsw/ffw, but in truth ALL dives are deco dives. All that happens is that our ascent rate must be slower if we have taken up more inert gas. We accomplish this in deco diving by making "stops"; we could do the same thing with graduated ascents carefully calibrated as to their speed, but its far easier to say "stop at 20' for 10 minutes" than to say "ascend from 30 to 20' at a rate which requires 10 minutes to complete." Yet the latter, arguably, is the "better" procedure, in that it would lead to a cleaner ultimate result.
The overpressure of Nitrogen (and/or Helium) in your tissues comes out over time during your ascent and on the surface. The rate at which it comes out of each of your tissues' solution is largely controlled by the difference in saturation levels between the various tissues between the one with the gas, and your lung's airspace. For most tissues this means the blood - which has a very "fast" half-time.
So, if you breathe pure O2, you will in a short period of time remove almost ALL of the dissolved nitrogen in the blood. This greatly raises the differential between the blood and your other tissues. The result is that the nitrogen will leave your body faster.
This is WHY technical divers use high-PPO2 gasses for the in-water part of their decompression, up to and including 100% O2 at shallow stops.
But you can take advantage of this too if you are at risk, without going back in the water. Simply grab the O2 and breathe it at 100%. You will flush all the nitrogen from your blood, which will then "pull" the remaining nitrogen from your other tissues. If you have an excess amount of it in your body - enough to have caused bubbles to form and grow in size - you MAY be able to pull enough of it out of those tissues, or even out of the bubbles themselves and back into solution, to prevent or arrest an incipient DCS hit.
But let's say you fail and get hit anyway. Well, not only did you at least try, but you have lowered the remaining overpressure in your tissues. As such the hit SHOULD be less serious than it would have otherwise been.
There are essentially no contraindications to an hour or two worth of surface O2 if (1) you are in good enough health to be diving in the first place, and (2) you have a reason to believe you're at increased risk. Serious pulmonary issues don't begin to appear for 12 hours or so, and you can even stave THOSE off with air breaks. Even if you DO get put in the chamber subsequently for a hit, the amount of O2 "whole body" clock time you accumulate breathing the pure O2 for two hours is not going to be a factor in limiting your treatment.
An hour's worth of pure O2 on the surface will wash out an enormous amount of residual nitrogen. If you believe you're at risk grab the bottle - its cheap, literally a couple of bucks worth of the gas - and it might save you a chamber ride.
