since ozone is a GHG then how ozone layer protects us from UV rays?

Tropospheric ozone is a short-lived greenhouse gas, which decays in the atmosphere much more quickly than carbon dioxide. This means that over a 20 year horizon, the global warming potential of tropospheric ozone is much less. Because of its short-lived nature, tropospheric ozone does not have strong global effects, but has very strong radiative forcing effects on regional scales. (Ozone acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth). Ground-level ozone pollution (tropospheric ozone) is created near the Earth's surface by the action of daylight UV rays on the ozone precursors.

The highest levels of ozone in the atmosphere are in the stratosphere, in a region also known as the ozone layer. This ozone layer is useful, preventing damaging ultraviolet light from reaching the Earth's surface, to the benefit of both plants and animals.