It works for me now, not all ROMS work just as good, but PAL vs NTSC did the trick for e.g. Tekken 3.
Also, I had to unplug my MX4SIO to have a higher chance of the USB being read by OPL Beta 1964. Had to restart the PS2 and retry opening OPL often, frozen screen a lot. Not sure if this is OPL...