How difficult is it to specialize in the Army?

Duty station location has little to do with going to the 66S Critical-Care course. You will spend 2 (3 years if you are OCONUS) years on a medsurg floor and about 6 months before you PCS you’ll drop a packet to go to the ICU course. Almost everybody gets picked up for the course. It is true that around the 8 year mark you’ll get pulled from the bedside and put in an OIC position. If you specialize ie. going to the ICU course it will delay getting pulled from the beside. 
 

Class ranking has nothing to do with duty station location for nurses. It’s first come first serve after you pass your boards.  A Brigade Nurse Counselor should be your primary POC for all these questions! They are basically a guidance counselor for nurse cadets. Your cadre will be little to no help since they are most likely all combat arms follks. Hopes this helps!

You can apply to the course after 1 year. You do not have to wait until you PCS. I work at a Army hospital as a civilian OR nurse and we have several RN’s who worked the floor here and went to the course after 1 year. After they finished the course they returned here. 

L&D and OR are the only specialties that are TDY and return. The critical care course is PCS enroute. 

Thank you so much to both of y’all. Super helpful. I am meeting my Brigade Nurse Counselor sometime this semester, still TBD LOL. Rhody, you seem super knowledgeable, would you mind if I asked you one more question? Say I were to get my nurse practitioner online at my first duty station instead of wanting to specialize.  Is it easy to make the MOS change from 66H to FNP(66P) or PMHNP(66R)?

You will have to apply to have your AOC

TDY and return depends on your command.  Some commands will allow it and others won’t, it depends the nursing leadership and hospital staffing. 
 

I’m pretty unfamiliar with online NP programs but I imagine they do require some in-person cliniclals which might be tricky working the floor. Say everything works out and you get your NP, I believe the process for switching AOC is still the same. There’s a section on the HRC Nurse Corp home page that walks you through it. It’s basically a TON of paperwork you have to get your nursing leadership (up to the CNO) to sign. I haven’t heard of active duty Med-Surg LT’s working the floor doing this, but have seen some CPT/MAJ in support roles get their NP on the side. Your BNC will probably try to sell you on LTHET to become an NP but you’ll have to wait until you’re a CPT to apply.

jeckrn1 is correct it depends on your CNO if you can bounce early to go to a course or do TDY in return.... but staffing is always an issue on medsurg floors :/ 

It's been a little while since I went to the ICU course (2012) but I was able to get into the ICU course quicker than my peers at other duty stations because I was stationed at Fort Sam.  Generally they want you to finish your 3 years time on station and then attend the course in route to your next duty station. My first duty station was at Fort Sam Houston and I was able to attend the course after I was working the floor for 18 months. Again this was back in 2012 so I am not as up to speed on the current situation regarding the specialty courses and how long it is taking new ROTC grads to attend.

Lots of good info above! I was fortunate to attend the CC/EN course in 2018 as a reservist which is super rare. We had a class of 30 at the start. They have like 4 courses per year. Course was very difficult but I learned a ton. Goodluck!