Sigma Sigma Phi (SSP) = National Osteopathic Service Fraternity, you need to be top 30% of the class, but the main emphasis is on service. Students are inducted into this society by fellow students who are already members. Most DO schools have this society on campus.
Psi Sigma Alpha (PSA) = National Osteopathic Scholastic Honor Society, you need to be top 15% of the class by the end of 2nd year of Medical School. No service requirement, purely academic. Students are inducted into this society by senior faculty members and the dean of the college. Only a few DO schools have this society on campus. ( KCUMB, UNECOM, and a couple of others)
Hope this helps