In aqueous solution, the nucleophilicity depends on the factor that which can make stronger H-bonding as the one which make stronger H-bond will be more sterically hindered by solvent molecules, and its electrons are less available to donate to other atom, that makes it weak nucleophile. So between RO⁻ AND RS⁻, RO⁻ is weak nucleophile as O is more electronegative, smaller in size than S so make stronger H-bond and weak nucleophile .
In DMSO, DMSO is aprotic solvent, so here no concept of H-bonding is required, RO⁻ is stronger nucleophile as O is more electronegative, smaller in size than S can donate electrons easily .
