If you all decided to stop accepting the mandatory OT they would let you work short, and let you think it was your fault. It's not, it's the hospital's responsibility to provide adequate staffing. If they really need the MOT then the supervisors and the managers should be the first ones mandated because that responsibiltiy is in their job description, it's not in yours.
Just the fact that as a staff nurse someone would try to pull off telling me that lack of patient care and safety was my fault, when it's clearly THEIRS was enough for me to walk away from MOT. Their ace card has always been the guilt trip, and we always suck it up like a Hoover Deluxe, so
And FYI I am now a supervisor, and yes, I have had times when MOT would be appropriate, and I have been the one to stay and help out. I can't see any other defendable position.