RE: "What do these initials stand for please, so that I may look them up? OIS / COT / ADT"
OIS = Officer Indoctrination School if you're in the Navy. It doesn't ring a bell for anything in the Army that's medical or associated with personnel actions that an HPSP officer might be involved in, sorry.
COT - Army = Continuation of Tour, as in: you've been assigned to Germany for a three year tour of duty, and now you expect to receive orders to return to the Continental United States (CONUS) on your Date of Eligibility for Return from Overseas (DEROS), but you really love partying in Stuttgart during Oktoberfest and want to stay, so you ask for and get a COT.
ADT - Army = Active Duty for Training. This is one form of active duty that can be served by personnel in the Army Reserve or National Guard. This is what the 45 days in the summer (school schedule allowing) for HPSP is. WHile on ADT, HPSP participants receive full pay and allowances commensurate with rank and length of sevrice (for HPSP contracts = O-1 with 0, or Second Lieutenant with no time in service).