I wish that was the case for the Army. If I was getting looked at just one year earlier than I am, I could apply for pro-rated retirement under TERA. The authority for that ends right when my board is meeting. The math works out that I'd be forced out at something like 17 years, 9 months. The Army held a continuation board last year for O-4s twice passed over, but I don't know how many were retained or if they'll have another one by the time my turn comes.
You don't mention the precise dates, but keep in mind that the typical wording of the promotion process is "... O-4s who are twice passed over for O-5 and not continued on active duty will separate by the first day of the seventh month after the continuation board results are approved." I'd have to look long & hard to find the right portion of the federal law, but I think it's the same for all the military services.
As close as you are to 18, you'd be offered TERA or continued for two more years. It might be two more years in Korea, but you'd be continued. If the Army tried to do to you what was contemplated during the first OSB, then the New York Times would have a delightful editorial month of mobilizing half of Congress to write letters to the Army offering helpful legislative solutions.
The vast majority of the OSB officers were over bodyfat composition, or failing the physical test, or had DUIs, or had letters of caution (or worse), or had even nastier black marks on their records. Some were punished for enlisted NJP mistakes made when they were barely out of recruit training. Unfair, borderline unethical, but legal.
Regardless of your rank, here's another thought from the financial side: if you're close to FI then it might not be worth gutting it out to 20 years on active duty (even from 18 years). You could leave active duty for the Reserves/Guard and drill for two more good years. When you reach 20 good years (and have your Notice Of Eligibility letter) then you could retire awaiting pay (or keep drilling). You'd get a pension at age 60 instead of in your 30s or 40s, but it would be at the maximum pay column for O-4s from the pay tables in effect during the year that you turn 60.
This would mean that your investments (and any income from a bridge career) would only have to bridge the gap from leaving active duty until you reach age 60.
Because Korea. After you reach 16 years of active duty the assignment officers feel that you're their b... er, I mean, theirs to send to only the most challenging promotion opportunities.
My spouse and I have tested this concept. In 1999 we reached FI solely on investments assets, and even after 9/11 we were still above the 4% SWR. At that point she had decided to leave active duty (bad assignment officer) because "It's only money". She officially separated with 17 years 11 months and 10 days and affiliated with a Reserve unit the next day.
Our assets would have easily bridged the gap from her separation until starting her Reserve pension at age 60. Better yet, our family quality of life quickly shot up above awesome.
If it's no longer challenging or fulfilling, and if the fun has stopped, I strongly recommend considring leaving active duty for the Guard/Reserve.
