OCPM means optimized CPM
OCPM is an advertisers purchasing model where advertisements are shown to people who will most likely convert. Facebook has rolled out this system at the beginning of 2013 and is designed to link online conversions to Facebook ads. With this system, actions that take place outside the social network can still be traced to specific advertisements. This might allow the system to replace old-fashioned cookies. The problem of cookies is that it is stored on a computer, when you are on your mobile phone or on another computer it is no longer possible to identify the user. With OCPM, this is possible, because Facebook knows who the user is, whether it's via a desktop or a mobile phone.