Actually, the OP-1 doesn't seem to send start messages, only continue, along with a song position pointer. Here's a read out using snoize MIDI monitor.
02:59:42.661 From OP-1 Midi Device Stop
02:59:42.813 From OP-1 Midi Device Stop
02:59:47.164 From OP-1 Midi Device Song Position Pointer 64 13
02:59:47.164 From OP-1 Midi Device Continue
02:59:47.611 From OP-1 Midi Device Stop
02:59:48.187 From OP-1 Midi Device Song Position Pointer 68 13
02:59:48.187 From OP-1 Midi Device Continue
02:59:48.884 From OP-1 Midi Device Stop
02:59:49.445 From OP-1 Midi Device Song Position Pointer 75 13
02:59:49.445 From OP-1 Midi Device Continue
02:59:50.121 From OP-1 Midi Device Stop
02:59:50.511 From OP-1 Midi Device Stop
02:59:50.640 From OP-1 Midi Device Stop
02:59:53.364 From OP-1 Midi Device Song Position Pointer 64 13
02:59:53.364 From OP-1 Midi Device Continue