I don't know much about the reserve side of things, but I can speak a little on active duty from my experience before my life as a CPA. Any officer designation in the Navy has a board process. They are highly competitive, or at least they were when I went through it. It took me about two years to get through the board process. There is a lot of paperwork involved, an aptitude test (OAR), as well as going to MEPS for your physical. When the board convenes, they select candidates based on letters of recommendation, GPA, aptitude test scores, and take into considerations any waivers needed to be fit for service. After you are selected, active duty candidates go to Officer Candidate School. Some designations like lawyers and doctors go to Officer Development School (which is a joke, they outrank their drill instructor and chief from day one). Supply Corps, a restricted line, goes to OCS with what are called unrestricted line officer designations (SWO, Pilot, NFO, etc). I think reserve has some other kind of commissioning program, not sure of the details. After an all expenses paid beach vacation to Naval Station Newport (LOL you do get to “play” in the sand a little), Supply officers go to supply school where they learn how to do their job, everything at OCS is more of a test to see if you can be broken down into nothing and built back up into a junior officer. It's a very mental process, the physical stuff isn't bad at all if you're in half decent shape.
One thing to consider is that designations can be changed. Everything is subject to the needs of the Navy, and things change throughout OCS and immediately after. A buddy of mine was there for Supply and had his CPA in hand. They redesignated him to SWO and he's still doing that today. Doesn't do accounting at all. I knew other people who had Pilot slots and ended up being redesignated to engineering, SWO, and NFO.
My piece of advice for those considering joining any branch of the military. Make sure that you want to be a sailor, soldier, etc first and foremost. Your specific job, comes second.
The military is full of highly motivated people who are there for the right reasons, and it's full of people who are there because they have nothing else to do and don't give two sh*ts about it. Please be the former.