What does OP stand for?

Oxycontin Perdue. These are the reformulated tablets. They reformulated them in 2012 to make them less easy to abuse by crushing and injecting. The old formulation had OC on them, to differentiate then new reformulated tabs have the OP. The new tabs resist crushing and still keep some of the time release properties if they are crushed and if you add them to water they turn to gel so they cannot be dissolved and injected