I use the string synth most (of the synths) but really spend 90% of my OP-1 time in the 4 track. I put it on an effects channel and then loop and manipulate stuff. I love the filter (I can’t remember the name, the one with the car driving fast) and the spring reverb. I just started getting more serious about messing with the LFO’s and have been happy with the results.
The endless sequencer applied to the string synth, slowed waaaaaay down in the four track is a pretty regular go to when I’m looking to start a new piece from scratch.
Also, if you like a bit of weird noise, try recording to a track from the microphone, while playing other tracks at high volume, essentially bouncing a track via the mic. It creates some weird digital artifacts that when you slow the tape way down sound super cool. I don’t have an example at hand. Your mileage may vary.
The newish volt synth is also great.
Last but not least - read the manual, I re-read chunks of it every couple of months and always find something new.