Order of activating ability in electrophilic aromatic substitutions
The most likely reason for this is that an $\ce{OH}$ group can act as an acid and exists in an equilibrium with the $\ce{O-}$ anion, which is significantly more activating.
$\ce{NH2},$ on the other hand, is not really that acidic. It does not benefit enough in this manner to be significant.
The loss of the anionic form is making $\ce{OR}$ less activating despite the inductive effect. $\ce{NR2},$ however, does not have as much to lose and only gains in activating ability.