Why does sound travel faster in Hydrogen than Oxygen?

The speed of sound depends upon two factors firstly the elasticity and secondly the density of the medium through which it is propagating. Usually, sound travels faster in liquids than that of in gases and faster in solids than that of in liquids. This means that the larger will  be the elasticity and the smaller will be the density then the faster will be the speed of the sound in the respective medium. Mathematically, the relation for the speed of sound is given as, 

$speed=\frac{elasticity}{density}$

As oxygen (air) and hydrogen have nearly the same elastic properties but the density of hydrogen is smaller than that of oxygen. Therefore, sound travels faster (approximate 4 times) in hydrogen than in oxygen.