So, what you're saying is oxygen and hydrogen will just naturally attach themselves to one another when they are close enough (I'm no chemist)?  There is no other reaction or force necessary?

Thank-you.

>So, what you're saying is oxygen and hydrogen will just naturally attach themselves to one another when they are close enough (I'm no chemist)?  There is no other reaction or force necessary?<

It's not quite like that. If you mix two parts gaseous hydrogen with one part gaseous oxygen the mixture will sit quite happily for a long time. Introduce a spark and the whole lot will explode with great vigour and you'll be left with water.

The point is that gaseous hydrogen and oxygen are both fairly stable, but it doesn't take a lot of heat to rupture the molecular "glue" and create free radicals which are extremely reactive. Once a couple of hydrogens have joined to an oxygen they give off enough heat to set in train a chain reaction that propagates with great rapidity - hence the explosion.

What Pete says is correct of course for normal laboratory work, where we routinely have access to molecular oxygen and hydrogen. (Mind you, if your oxygen happens to be ozone, you need not wait for a spark! Also, ozone or no ozone, if you have a reasonably good mix of say, 1 part of H2 to 8 parts of O2 by mass, it had better not be a large mass! Or it had better be a seriously strong container. The little "phew" noise that your chemistry teacher might show you when lighting a test tube of hydrogen gives you no idea of the violence of hydrogen/oxygen explosions, or even their burning.) 

But in space, given millions of years, and with all sorts of wavelengths of ultraviolet and even more energetic photons ionising molecules and producing free atoms, you would expect very few oxygen atoms not to find partners pretty soon. What with the overwhelming majority of atoms in most parts of the universe being hydrogen, it is a safe bet that a search for molecular O2 would have to be intensive, and you had better get there quickly before some hydrogen atom beats you to it!

Jon

Before submitting my answer I tried to find out whether H2 and O2 could react to form water non-explosively. What I mean is: if you left them sitting in a vessel for long enough, would you find it bereft of gas with a little puddle of water at the bottom? (Or, more likely, full of gaseous water... but you know what I mean).

After all, methane doesn't survive for long in the atmosphere - surely H2 can't stay round for long in O2? Or is it bang! or nothin' ?