The old way ... your PGY-1 year is "Intern" and then you begin 2 year residency in internal medicine (PGY2 and PGY3). Total years = 3. Under the old method, you could have done your internship either in a traditional intern, specialty-tracked internship, and speciality emphasized internship ... the difference which were confusing to everyone.
So now to make it less confusing, the AOA has changed it to be more similar to the ACGME style
Option 1: Internship is now linked with residency ... so instead of PGY1 (intern), it's PGY1 (resident).
Option 2: Osteopathic internship that is an analog to the transition year internship seen in the allopathic side ... for those whose residency starts at the PGY2 level (radiology, PM&R, etc)
Option 3: Your traditional rotating internship
If doing an osteopathic residency, the time you spent should be the same as your allopathic counterpart. If you want to do an allopathic residency and wish to be licensed in the 5 states, then you can either petition the AOA to approve your ACGME PGY1 year as an AOA-approved year, OR you can do an osteopathic internship and then go into an allopathic residency (and adding an extra year to your postgraduate training)