Actually...different programs differ
Oral surgery: is normally 6
(To get the DDS/MD it's six, but the certificate without an MD is 4. See UCLA and UCSF oral surgery programs). Normally one or two person gets into this program per school.
Ortho: is normally 2-3 years
(most schools offer it in 3 years and give its graduates a Masters degree. However, I know that U of Louisville is a 2 year program with a Masters degree option)
Pedo: Same as ortho..don't go to UCSF...they force 2 out of 4 residents to go into the Pedo/PhD program which can take from 5-7 years.
I know these things...trust me...2 dentists in my family. I am a med. student though. You know typical asian family..everyone is in the health care profession...