You inquired about the Warrant Officer 255N career path and now you are interested in O-grade scenarios?
Commissioned Warrants and O-grades share a lot of the same under statutory requirements even though their ascension processes differ. Every new Soldier or servicemember incurs a eight-obligation so if you serve three years on active duty; the remaining five would be in the IRR/National Guard or Reserve unit.
The Army does not send officers to grad school. Its a process in which an officer applies for and has to meet a requirement after. Sometimes the requirement runs concurrent with their current obligation.
Unlike every prior service member, the length of time from service and the component re-entering (e.g. guard to active, active to guard) will determine if there is any rank reduction. The officer ranks do not have the same flexibility as enlisted. Basically if your obligation runs out and you resign your commission; the processing for re-entering is like searching for needles in a haystack. Many former officers have reverted to the enlisted ranks because of that so they can achieve a 20-year retirement. Usually when an officer leave the military service; it is for good.
I hope my comments helped