RO- as leaving group in saponification process
$\ce{OH-}$ of course can, and will, be a leaving group here. The reaction is in equilibrium, $\ce{OH-}$ adds and leaves very many times, but that is a dead end as it does not drive the reaction forward. $\ce{RO-}$ can also be the leaving group - the difference between the basicity of $\ce{OH-}$ and $\ce{RO-}$ is not so great that it does not happen. Even if it happens one time in ten thousand, when it does the cascade it sets off makes the reaction irreversible so the reaction proceeds, if slowly, and the salt of the carboxylate is produced.