due to small size and high electronegetivity of oxygen, it can form p pi-p pi bonds, as a result oxygen exist as a diatomic molecule(O2) and has less intermolecular force of attraction. therefore O2 is gas at room temperature. but on the other hand sulphur is larger in size than oxygen and less electronegetivity than oxygen, sulphur can't make p pi-p pi bond multiple bond. instead it prefers to form S-S single bond and as a result, sulphur exist as octaatomic (S8) and has high intermolecular force of attraction. Therefore sulphur is a solid at room temperature .