As in well known, a Hankel operator Hf densely defined on a Hardy space H 2 on the unit circle T can be extended to H 2 if and only if its symbol f∊BMO(T) [15]. In this paper, we show that in an appropriate form this result holds for all finitely connected domains in ℂ with Jordan boundaries. This requires developing a conformal invariant notion of BMO for such domains and a generalization of the classical result of F. Riesz concerning factorization of H 1 functions into two H 2 factors.