Why Sulphide ion (S2-) is less basic than oxide ion ( O2-)?

SOLUTION: Sulphide ion (S^2-) is less basic than oxide ion (O^2-) due  to its larger size. the oxide ion has a higher charge density than the sulphide ion hence the small H⁺ more attracted toward smaller ion (O^2-) .

Which is more basic oxide ion (O-2 ) or hydroxide ion( OH- ) and why ?

SOLUTION: The basic strength of base primarily depends on the ability of a species to attract H⁺, which is also depends upon electrostatic interactions. The smaller and more highly charged a negative species is, the stronger the attraction for the small, positive H⁺ ion. For example, O^2- has a higher negative charge than OH^-, and as a result it is a stronger base. One does not normally expect reactions that take place in aqueous solutions to produce oxides as a result of the basicity of O^2-. Ionic oxides normally react with water to produce hydroxides:

                                           CaO + H₂O → Ca(OH)₂