If you had an intersteller cloud of Hydrogen and oxygen that was somehow dense enough to react to create water then you would be right but gass clouds in intersteller space are so rarified they would equal a good vacum on earth so in reality there would be no reaction. In space a cloud of free oxygen would be rare anyway. Oxygen is almost alwasy found in combination with something else because it is so reactive. If you did have the hypothectical cloud you talked about it would need no spark, hydrogen and oxygen would react with each other as soon as the pressure reached high enough levels from the action of UV light.

Moon