Dear Mr Cat
Well, first of all are you a US or Canadian Dental graduate? If you are, then yes Orthodontics is like you said pretty competitive, with regards to availablity of residencies, and yes you have to be on top 10, but this varies from programme to another, some programmes take top 5, some take the top one third of the class, so it vairs but generally, it is a really attractive career, I think also your Board result should be competitive, In case that you are a forign dental graduate, then first you need to get a DDS or DMD and I don't know about the rest, I think you can qualify
Hope that was helpful
Khalid