\(RX + \mathop {O{H^ - }}\limits_{Nucleophile} \to R - OH + {X^ - }\)
\(O^{-}\) stronger nucleophile than halogen so, easily replace the weaker nucleophile. Nucleophiles are either negative charge or lone pair of electrons bearing species, e.g. \(O H^{-} ; \ddot{N H}_{3},\) etc