I have an alternative suggestion for removal of the back panel.

I also had difficulty removing the back panel using a jimmy (and a guitar pick), so I stuck some heavy-duty tape to one end of the panel and gave the tape a sharp pull perpendicular to the surface. It came off quickly and easily without any damage to either the panel or the body of the OP-1. There was some minor tape residue left over but it cleaned up without any issue.

I used cloth backed electrical tape, but I imagine that any strong non-permanent tape would do the trick.