Actually, OMC is one of the many names used by the company that is most commonly known as Arcadia Machine and Tool, or AMT. They were known as OMC or TDE in their earlier years.
It has been many years since I sold my AMT single-action BackUp, so I am afraid I no longer recall the field-strip instructions. I think you had to drive out a roll pin and then tap out the breechblock from the bottom of the magazine well, but I could be mistaken. Try to Google for 'AMT Backup disassembly' or something like that.
Snohomish County, Washington USA