Why does molecular oxygen absorb in the IR range?

I am studying spectroscopy and know that $\ce{CH4}$ and $\ce{CO2}$ are known as greenhouse gases because they are IR active. This is because they have asymmetric stretch or bend vibrational modes which result in electric field changes and involves emission or absorption of IR light.


But, $\ce{O2}$ is a symmetric molecule and should be IR inactive. I am wondering why there are still strong $\ce{O2}$ absorption bands in the IR range if it is expected to be transparent to IR light?


