ENLIGHTEN ME, PLEASE!

You ask much but I'll try my best. The internship is in addition to a residency for board certification. Every state is different about licensing but in Oregon (where I obviously am) the state board requires only one year post-grad training. However, good luck getting patience or mal-practice insurance! The internship is used by many residency programs as a PGY-1 (post-grad year one) and thus you would start on year two of residency training. Not all do this and you would need to speak to residency directors to find out each program's specifics.
Part two
Find a D.O. (preferably in your area of presumed interest) and contact their secretary or whoever handles their appointments and explain to them politely that you are an aspiring medical student, you are interested in osteopathic medicine, and would Dr. so and so be willing to give you a few minutes of their time. Do this till you get a response. At your meeting you ask questions showing interest in Osteo-med and then ask if they would mind you shadowing. If they say "No" then politely ask them to refer you to another doc that they think could.

BE persistent just short of being a pest and you should find you efforts rewarded.