${\mathrm{O}}_2$ molecule is paramagnetic because it contains

two unpaired electrons in $\stackrel{*}{\mathrm{\pi }}$$2{\mathrm{p}}_{\mathrm{x}}$ molecular orbital

two unpaired electrons in $\stackrel{*}{\mathrm{\pi }}$$2{\mathrm{p}}_{\mathrm{z}}$ orbitals.

two unpaired electrons in $\stackrel{*}{\mathrm{\pi }}$$2{\mathrm{p}}_{\mathrm{x}}$ and $\stackrel{*}{\mathrm{\pi }}$$2{\mathrm{p}}_{\mathrm{y}}$ molecular orbitals

two unpaired electrons in $\stackrel{*}{\mathrm{\pi }}$$2{\mathrm{p}}_{\mathrm{y}}$ molecular orbital