Absolute nonesense to suggest that simply turning up for an OCU ties you into a ROS.
You have to be offered the chance to sign it prior to starting training. If you refuse to sign it, training can still commence if it is judged that you are unlikely to leave anytime soon.
I know of one FC who PVRd before his ROS was up and, after mentioning a solicitor, walked away into civvy street.
Bottom line is that the AP that holds reference to ROS does not stand up under civil law. Instead, the powers that be in the ivory towers rely on personnel being intimidated into 'honouring' the agreement.