Honestly, focus on getting commissioned first. Selection board stats are going to be a world different by the time you are eligible for O4, assuming you get selected for DCO.
BT
You are eligible for promotion at 9 years
commissioned service. When I was commissioned, I had 7 years total qualifying service. When I made O4 at 9 years commissioned service, I had 16 years total years of qualifying service.
If you 2xFOS for O4 you are out unless you get continuation. Read
here. You can also call PERS-801 (number on the web site link).
Here is what it may look like for you assuming got commissioned this month. You are retirement eligible at 20 total qualifying years of service. At least you will have 4 years when you start. Most have zero.
So, if you 2xFOS for O4 you will do so at between 13 - 14 total years of qualifying service and will get the boot. The two people that I know who 2xFOS for O4 were both prior enlisted and were already at 20 total years of qualifying service, so they retired as LT/O3's.
Going enlisted is not going to help either as when you hit High Year Tenure (HYT) in for your rank, you will get kicked to the Volunteer Training Unit (VTU) which is the same as SELRES but with no pay.
