I am quite new to ot, and have connected my a4 to it with thru machine. If i hit play on ot both sequencers run in sync, but if i hit stop on a4, often by accident, i cannot get them to sync again without stopping it all on the octa and hit play again. Is there a way to be able to stop and play the external machine and it catches up in sync again?
The simple way is to stop / start on the OT. If you time it right, you can do that live and create an “interesting” turnaround in the track. 
This thing does ‘resync’: http://www.midiclock.de/midiclock.php#
Not sure how the OT would react to it, as the OT likes to be the master.
I remember reading about some kind of big red resync button device.
Ah, here it is: http://mungo.com.au/syncZ.html