It would be interesting to see if it would go exothermic or not. On the one hand you would suddenly have a bunch of free hydrogen atoms, presumably becoming H2 as they expanded. The pressure wave would be caused by sudden decrease in density (phase shift from solid to gas) but and in the process the new molecules would probably briefly become liquid and then gas. There would not be enough oxygen in the center of that expansion to convert to water, but on the edges of the expanding pressure wave their would be. One possible outcome would be that on the surface of the pressure wave, oxygen would combine with hydrogen to form water, but that water would then insulate the hydrogen behind it from any oxygen, stopping any further reaction. At some point thought he heat going into the phase shift is going to come out of the water, which will force it to freeze, and the as the surface area of the pressure bubble expands, its pressure will decrease until it is in equlibrium with the frozen water's surface tension.

One possible outcome then is than a conversion would result in a 'pop', and the sudden appearance of an ice sphere which is filled with pure hydrogen.

Fee hydrogen (atomic hydrogen) releases a tremendous amount of energy when it reacts to make molecular hydrogen (H2). Far far more than reacting with oxygen.

So the oxygen part is pretty irrelevant when analyzing this.