Both oxygen and sulphur exist as diatomic molecules  $O_2 \&S_2$ in gaseous state but at room temperature oxygen remains as diatomic molecule while sulphur converts into $S_8$ rings. This is because

When solid sulphur is heated it converts into vapour by absorbing energy to break strong 4 sigma bonds in $S_8$  ring. In $S_2$  molecule a weak  $\pi$ bond is formed, due to less effective lateral overlap of p  orbitals between bigger sulphur atoms. At room temperature when  $S_2$ molecule converts into S8 rings the 4 weak, $\pi$ bonds breakup and form strong  sigma bonds liberating energy thus giving stability to  $4S–S$ molecule.  $O–O$ single bond is weaker due to repulsion between non–bonding electron pairs on small oxygen atoms. But the  $\pi$ bond is stronger in  $O_2$ due to effective overlap of P-orbitals in $O_2$  because of small size of oxygen atoms. During conversion of  $S_2$  molecules to $S_8$  rings though paramagnetic character decreases  and molecular size increases they have no role in the stability