Starfleet, as well as its Academy, seem to be set-up much differently than the US Military.
No officer in the US Military is required to attend his/her branch's Academy. In fact, it's damn hard to get in to the Academy. I believe most require a letter of endorsement from at least one of your CongresCritters, if not more, and I believe a Senator is preferred. Officers in the US go through OCS, or ROTC, and I believe they all need to possess a college degree to be formally commissioned.
Now, it stands to reason that Academy graduates would get better assignments right out of the Academy, and continue to do so until they screw something up majorly.
I don't know about Army doctors, but I believe all three JAG Corps require Basic of their Officers, and while they're not commissioned as Line Officers, they do have full authority over lower ranked officers based on assignment ranks. Certainly any non-Flag Officer should be addressing JAG's good ol' Rear Admiral AJ Chegwidden as "Sir," even if they be a Line Officer. But, his authority over a lower ranked officer not in the JAG would be limited.
As for Starfleet, there were several "crewmen" and NCOs who established an enlisted, non-Academy program. Simon Tarsus, along with O'Brien's reconned backstory established this.
There has never been any indication of a Reserve or National Guard type commissioning.
It seems like the Academy is a four year college as well as an OCS.
This is partially true. In the Army, most officers come from ROTC with about half coming from both the Academy and OCS. OCS is mostly for prior service people who started as enlisted and decide to become officers at a later date. This isnt always the case though. There are also ways for people to become officers through OCS with only 90 credits, but this is rare and there are alot of restrictions.
Now while officers aren't required to attend the Academy, in the case of the Army (and the other services as well) there is a training phase that occurs right after you get the commission. In the case of the Army, its called Basic Officer Leader Course part II and Part III. At this point you are integrated with OCS, Academy, and ROTC grads and essentially learn your job. It is in this particular area where there are alot of similarities between how Starfleet Academy works and how the US Army works
In my case, I'm becoming a signal officer so I would head to BOLC II for basic leadership and tactics/infantry stuff and then Signal BOLC III in Fort Gordon. In the case of direct commission officers, they do not have to head to Basic. As a matter of fact, the only officers who ever have to go to basic are OCS officers. It's optional for ROTC officers.
Anyways, from what we have seen of Starfleet Academy it really doesn't have any of the characteristics of OCS (in which case, every Academy graduate would have to start out being enlisted) and looks alot like you're regular old service academy merged with the post commissioning training. The only difference is that all officers are required to attend Starfleet Academy and it looks like branch specific training occurs on site as opposed to further down the line
EDIT: Something that I should mention about direct commissions is that all of the military doctors, nurses and lawyers I've met did not enter the military though direct commissioning. Most of them either commissioned into a traditional branch and then entered into a program that allowed them to get their graduate degree when they became a captain. Others commissioned directly into the JAG, Nursing, or medical corp. in the case of the doctors and lawyers they would then go on to medical school. Once they finished med school they would rejoin the Army