TL;DR: When slaving OP-1, it receives MIDI clock, and its BPM counter indicates it is synced. BUT, internally, OP-1 is NOT following the clock. Teenage Engineering should fix this.
Here is how I came to this conclusion.
I sync a lot of hardware to Ableton.
I usually set a negative MIDI clock sync delay in Ableton’s MIDI settings.
I did this for the OP-1, as I normally do.
To test my sync, I recorded the OP-1’s metronome into an Ableton track with monitoring OFF of course, so as to avoid Ableton’s latency (a huge topic).
I noticed my MIDI clock sync delay settings were NOT being used.
I then set extreme amounts of MIDI clock sync delay and did the test again.
There was no change.
OP-1 receives start and stop and clock. OP-1 only uses start and stop. OP-1’s BPM counter shows you it is syncing, but internally, OP-1 is only starting and stopping; it is not following the clock.
This is also why OP-1’s clock drifts.