I looked at the Roland M-OC1 manuals available on this page: Roland Manual Archive. It looks like the Vol&Hold setting needs to be either "on" or "vol" for the unit to obey incoming volume messages. Try checking that setting.
I looked at the Roland M-OC1 manuals available on this page: Roland Manual Archive. It looks like the Vol&Hold setting needs to be either "on" or "vol" for the unit to obey incoming volume messages. Try checking that setting.