Can a prior-service Marine, current National Guard, become a Marine Officer?

Marine, and I address you as such whether you are currently wearing the USMC uniform and formally part of the USMC by contract, or part of a sister-Service and simply a Marine inside because you have felt the transformation...,
Yes, you can become a USMC officer from the street, as a civilian, or from another Service, or from within the USMC (or USMCR, depending on your path and desires).
Your first steps beyond investigating the USMC officer commissioning process on the various USMC websites (mostly run by the recruiting command MCRC), is to call or physically visit a Marine Officer Selection Officer (OSO).
The OSO will be able to guide you based on your particular circumstances, because there are many variables here, such as:
1. When does your current contract end?
2. Which Service are you currently in?
3. Do you already have a college degree, or are you close to getting one?
4. are you trying to become a warrant officer or a line officer (lieutenant)? (Actually, while that is common wording to differentiate our USMC warrant officers from non-warrant officer...officers...it is actually incorrect because ALL USMC warrant and chief warrant officers are Officers of the Line, just like lieutenants and above, and ALL USMC officers of the line (which is all of them!) are capable, and in fact expected to, of assuming command and leadership responsibilities regardless of the unit or MOS...to lead Marines in combat...)
5. Are you currently in any other Service's officer candidate program?

Etc.
These are all questions you will need to discuss with the OSO to find out if, or how close, you might be to obtaining a USMC officer commission.
Semper fi, and good luck, WKB

I was enlisted (MC) for 6.5 years, in that time I earned my diploma. After I was done I applied for OCS under the OCC program. If accepted to the school, it is a direct commission upon completion of OCS. Then the follow on stuff is very much the same (TBS, MOS, etc).

“I’d rather be an outstanding Sergeant than just another officer.”