Why is ozone layer so important when oxygen atoms absorb UV Rays?
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.
When there is so much oxygen present in atmosphere how will UV Rays reach us?
Oxygen has a low cross section to UV radiation. It absorbs some, but not very much.
Are we concerned about oxygen being depleted?
Not at all. Suppose the ozone content did become rather high compared to nominal. This would increase the rate at which ozone combines with monatomic oxygen to form to diatomic oxygen molecules. Ozone is a transient component of the atmosphere. It is constantly being created but is also constantly being destroyed.