Welcome! I have not owned an OP1 for a long time, but I assume you have tried it after turning on the MIDI Controller Mode in the OP1’s settings? If it does not work in that mode, then there might be something incompatible with the OP1’s communication protocol. Also, I assume that you are referencing the original OP1 and not the OP1 Field? My understanding is that the OP1 Field operates as a MIDI host. The MM operates as a MIDI host and generally you cannot directly connect two devices acting as MIDI hosts together via their USB ports.
There is a thread devoted to working MIDI devices, but no mention in that thread of anyone connecting an OP-1 to MM. Working MIDI devices