What happens if I’m on OB Medevac and we PCS during that same period?

Home leave, transfer, curtailment, authorized voluntary evacuation or ordered mandatory evacuation orders supersede all other official travel (including medevac) and travel must occur on such orders (16 FAM 317.4b). Therefore, HSTA per diem would curtail your OB Med per diem and you would proceed on HSTA as soon as the Direct Hire PCSed.