Oxygen due to its small size and high electronegativity form pπ-pπ multiple bonds. Hence oxygen exists as a diatomic (O2) molecule.
On the other hand sulphur due to its large size and low electronegativity, do not form pπ-pπ multiple bonds. Instead it prefers to form S-S single bonds.
Further , it is exist as S8 molecule and due to large molecular mass of S8 unit, the van der Waals forces of attraction holding S8 molecules together is very large and as a result sulphur exist as solid at room temperature.
Final answer : It’s only due to strong van der Waals forces of attraction.