Why is ozone layer so important when oxygen atoms absorb UV Rays?

Oxygen molecules absorb UV Rays and are converted to elemental oxygen atoms. These elemental oxygen atoms combine with molecular oxygen to go give ozone .

When there is so much oxygen present in atmosphere how will UV Rays reach us? Are we concerned about oxygen being depleted?

The cross section of ozone to UV radiation is many, many orders of magnitude higher than is the cross section of $$\text O_2$$ to UV radiation, so very much higher that despite the paucity of ozone molecules compared to diatomic oxygen molecules even in the stratosphere, it is the ozone molecules that tend to absorb incoming UV radiation.