Usually dental schools want you to shadow and get an LOR from a general dentist - they want to make sure that you understand what general dentistry entails (lots of people want to specialize but don't make it). But if you get an LOR from an orthodontist in addition to an LOR from a general dentist, that's great!
A lor from an Ortho is just fine! As long as you have shadowing hours with a GP, you'll be golden. Orthodontists can attest to your character and ability to handle dental school just as well as any other dentist or specialist could, because they've been through it.
I would get a LOR from an oral surgeon so he can attest to your disdain/ dislike of blood. That way they will know you are meant to be an orthodontist.
Just being a smartass here, a running joke during my oral surgery residency of the past.