Ethyl acetate in which the alkoxy oxygen is labelled with ${}^{18}\mathrm{O}$ is hydrolysed with aqueous NaOH. The ${}^{18}\mathrm{O}$ label is found in

Hydroxide adds to the acyl carbon and the tetrahedral intermediate expels the alkoxide, so the acyl-oxygen bond is cleaved. The labelled alkoxy oxygen stays attached to the ethyl group and appears in ethanol, while the acetate formed contains only unlabelled oxygens.