  We show that if two Hopf algebras are monoidally equivalent, then their
categories of bicovariant differential calculi are equivalent. We then
classify, for $q \in \mathbb{C}^*$ not a root of unity, the finite dimensional
bicovariant differential calculi over the Hopf algebra $\mathcal{O}_q(SL_2)$.
Using a monoidal equivalence between free orthogonal Hopf algebras and
$\mathcal{O}_q(SL_2)$ for a given $q$, this leads us to the classification of
finite dimensional bicovariant differential calculi over free orthogonal Hopf
algebras.
