My setup is OT + A4 and I was also scratching my head about this for a while, but I think I have a working method now.
Basically what you want to do is program the arrangement in OT and have it control all your other Elektron boxes. You need to enable sending program changes in OT and enable receiving program changes for your other boxes. This means OT will send transport, clock and program changes to your other devices. Then you just need to make sure that patterns match between your devices and everything should just work.
Another way, which I tried first, is to only send transport & clock from OT and program the same arrangement on the A4. This also works. Both methods are a bit cumbersome to set up, you have to be careful everything matches, but should be rock solid in practice.
The advantage of the first method is that you can have stuff like infinite loops in the OT arrangement and manually jump out of them, or jump around the arrangement in general. This unfortunately doesn’t work in the second method, since OT’s Song Position keeps advancing instead of jumping when you jump around in the arrangement.
The advantage of the second methods is that you can use each device’s song mutes. In the first method, you would need to copy patterns in order to make versions with some tracks muted. Which method is better depends on your workflow and what type of music you are making.
I would be happy to hear about other solutions to the problem, as neither of these is ideal for all cases.