Some dental schools offer master degree in oral biology at no additional cost. One of my dental classmates got his MS and DDS degrees at the same time. He later got accepted to a very good paid perio residency program.
When I did my ortho residency at another dental school, I saw a few DMD students who took the same classes, which were required for MS degree, with me. Most of these students specialized after dental school. Having an addtional MS degree definitely helps increase the chance of getting accepted to residency programs, especially for ortho and perio.