Why Is Helium Used In Balloons Instead Of Oxygen?

A balloon would sink if its contents are heavier than air.  Pure Oxygen has a density of 1.428 g/L.   Air is made up of 21% Oxygen, but the majority (78%) is Nitrogen, a lighter gas, at 1.251 g/L.  The density of Air is 1.296 g/L and is lighter than Oxygen.  

Since pure Oxygen is heavier than air, a balloon filled with Oxygen would sink because of its heavier contents plus the weight of the balloon.   It is also a safety issue because it is highly flammable.

Because Helium is lighter than air so balloon floats. O2 isn't used in anything because of it's high combustion rate.

Actually, neither is used in larger balloons.  Both are highly flammable, and oxygen is too dense to lift anything. Helium is used in small party balloons, because it is lighter than air. However, since we have used helium so much for unnecessary things, there is now a shortage of helium.

Helium gas is lighter

In addition to what these guys have already said, helium if inhaled is harmless to humans (unless in large amounts), whereas pure oxygen can be lethal. Oxygen is almost more expensive to purchase than helium....