wrote:

It seems like a waste to spend 8 years in the USMC accumulating rank
and grade, and then go all the way back to O-1, 0 years in the Navy.
Even if you serve your 8 years as a Marine aviator, is it possible to
preserve your rank and grade when you go into the Navy as an MD?

Your active service time accumulates regardless of service.

Going into the Navy [or Army, or USAF, or Coast Guard] as an MD
currently carries a minimum rank, I believe, of O-3 [Navy/USCG LT,
Army/USAF Captain], possibly higher depending on
specialties/certifications/etc.

Inter-service transfers, outside of your desired scenario, typically
are the subject of how your qualifications match the new service's
requirements, and some resulting negotiation; but for officers usually
results in a rank for rank crossover at the levels you're talking.
Enlisted can be somewhat more difficult, and are beyond your query, so
I'm not gonna venture into that morass of possibilities and
qualifications.

OJ III