If you wanted to enlist in the Navy back in the day, you just strolled into your local recruiter and chatted him (or her) up. You'd take the ASVAB (Armed Services Vocational Aptitude Battery) test, fill out some forms, take a physical, and off you'd go. The big determiners on what you did were the needs of the Navy and your ASVAB score, with added pluses for relevant experience. If your requested skill required that you'd have to learn a foreign language, you also had to take the Defense Language Aptitude Battery (DLAB) test.
Then it was off to the Recruit Training Centers, known throughout the Navy as "boot camps." There were three of them, RTC Orlando, RTC San Diego and RTC Great Lakes. If you had scored well on the ASVAB and there were slots open, you would go from boot camp to "A School" to learn the basics of a technical rating. If you couldn't get what you wanted, you would be sent to the Fleet as an undesignated seaman (deck), fireman (engineering) or airman (carriers). There you would take a correspondence course in the rating you wanted to get into, or "strike for." You would have to demonstrate your interest and ability to the Striker Selection Board in order to be transferred into that division. And, most important, you had to impress that division that you'd be a good addition, as nobody wanted to be saddled with a dirtbag.
For officers, there were several routes.
Oldest of all: The Naval Academy (USNA) in Annapolis, MD, the official brain washer of the officer corps, also referred to as the Boat School or the Chesapeake University of Nautical Technology. Their job was to produce the Kool-Aid Drinkers, the ones most likely to make a career out of it. You could enlist, then apply for the Academy. In that event, you normally had to first go to the USNA Prep School in Newport, RI. They had a five year commitment after being commissioned.
The Reserve Officer Training Corps (ROTC): They produced more officers than did the USNA. The ones on scholarships got all of the benefits of going to Annapolis (a free education), without the stringent 24/7 horseshit. If you wanted the free education, but you wanted to be able to drink copious amounts of beer and have frequent sex, you went to ROTC. 2 year ROTC grads had a four year commitment, 4-year ROTC grads had a five year commitment.
Both 4-year ROTC and USNA midshipmen went to sea serving as enlisted sailors during the summer between their freshman and sophomore years. Between their junior and senior years, they went to sea as junior officers. Between their sophomore and junior years, they went to play with the Marine Corps and/or the aviators.
Officer Candidate School (OCS): OCS was open to those who already had a 4-year college degree. The aviators had their OCS at Penascola, FL. Almost everyone else went to OCS in Newport, RI. This was a 19-week cram course that later was reduced to 16 weeks by increasing the class day from seven hours to eight hours. These were the "90 day wonders." A sailor in the Navy who had earned a degree could apply to go to OCS and many did. A fair number of enlisted from all services who had gotten out and then earned a degree went to OCS. The non-prior service OCs tended to have a few years of experience after college, some had master's degrees. OCS grads had a four-year commitment, though the ones who went off to nuclear school had a longer commitment.
Officers who went to USNA tended to stay in in greater percentages, as they were the ones who had been brainwashed. Those who went to OCS tended to leave after their initial service was done, as they saw the Navy more as "just another shitty job." OCS graduates were far less likely to have drunk the Kool-Aid than the others. OCS grads were the ones most likely to think outside of the box; they could be the superstars or the problem children of any wardroom. If you needed something done that was not usual procedure or might even be borderline on legality, you probably asked an OCS grad to do it. ROTC officers tended to fall in between the two extremes, but they tended to be more towards the OCS side of the spectrum.
Officer Indoctrination School (OIS): OIS was a four or six week school in little more than how to wear a uniform and not make an utter fool of yourself. OIS was only open to staff corps; people who had a skill the Navy needed, were not in line to command anything and were less likely to be sent to sea (baby supply pukes were sent to OCS). Doctors, nurses, chaplains and lawyers went to OIS. If the Navy needed them badly enough, they were given direct commissions, shown briefly how to wear a uniform and set loose. The results were often comical.
Limited Duty Officers (LDO): LDOs were senior enlisted who were directly commissioned as line officers. Because LDOs often had ten or more years of service, most retired soon after being promoted to lieutenant commander.
Post commissioning training for officers will be a topic for another day.