Hydrogen and oxygen in space chemistry tells us that when mixing hydrogen and oxygen the result is an explosion and water.

my question is: does this happen on astronomical scale? for example, did they ever observe planet-size hydrogen cloud collide with planet-size oxygen cloud to produce a huge explosion and planet made primary of water?
Hydrogen and oxygen only react when there is sufficient energy. For instance, the autoignition temperature of hydrogen at 1 atmosphere is 536 °C. This is why you can do that experiment with mixed hydrogen and oxygen in a balloon, that only explodes when you touch the balloon with a lit taper.

Space is cold. Molecular clouds have temperatures in the tens of degrees Kelvin. In the rare (if not impossible) case that a molecular cloud of pure oxygen did exist, and it encountered a molecular cloud of hydrogen, there wouldn't be enough energy for an explosion.