For me it completely depends on my mood and the track I want to make. That’s the cool thing about these two boxes… both are capable of a big variety of sounds.
I tend to use the OT for drums and the A4 for melodies and pads/drones. Usually about four drum tracks on the OT with sample chains, and on the often A4 a bass track, a pad (tuning the oscillators to chords, played with single notes) and two lead melody tracks. Then I sample some lines from the A4 into the OT either to make space for more stuff on the A4 or to manipulate the sounds on the OT in order to create additional elements. I love slicing, pitching, reversing melodies.
Since drums on the A4 sound so great I also often start with a drum beat on the A4 and sample it into the OT. Then slicing it up, make variations and glitches, applying effects etc.
I sequence the A4 from its own sequencer most of the time, cause it’s more direct (less time to setup - I’m lazy!) and I love the little keyboard.
Lately A4 and digitone share a lot of their tasks. Digitone is also great in every department, even better for ambient stuff imo
I know this trick from @sezare56, I used it a lot. Really powerful