Rylance has a performance on the day of the Oscars, so if Ronan does stay in NY, she won't be the only nominee to do so. Still, as others have said, it certainly isn't impossible for her to fly out the day of the ceremony and fly back the next day in time for their first preview. I wouldn't envy her flight schedule, but then again I don't have an Oscar-nomination and a starring role in a Broadway show to console me
I wonder how they would work that out with tech if Ronan does attend the awards. Would they have their final dress the day before the Oscars and take Oscar sunday off? Or would they have Ronan's understudy do the final dress (I also don't really know how tech schedules work on Broadway, in case it isn't clear).