The Octatrack can be a mixer/effects box pretty trivially. It sort of feels like a good first step on the way to using it as a live recorder/looper (which I think ultimately is a big sweet spot). The key to mixing/effects is the Thru Machine. They do pretty much what they sound like they do: they pass sound "thru" the Octatrack.
For this I grabbed a MIDI cable and plugged it into my eurorack. I wanted to grab clock and at first I thought something might be broken, but you have to go into MIDI settings (press Project, then go to MIDI) and turn on clock and transport sending, then it's nbd! I threw together a pretty quick semi-random drum loop and then plugged a stereo output pair into Octatrack inputs A and B.
From here, it was quick to get Octatrack sending the drum beat to the headphones and main outputs. To do this you set up a Thru machine on any track and in the setup configure it to listen to inputs A/B. Thru machines can apparently listen to a mixture of channels from inputs A-D though it's set up to favor the idea of A+B being one stereo pair and C+D being a second. In this case, I set the "How do I listen to A/B config" to "A B" which indicates listening to them as a stereo pair... and it started playing!
From here it's fun to mess with effects like reverb, filter, delay, etc. Even with a relatively constant repeating input there were fun things I could do with the scene fader. It immediately adds a new dimension of interactivity! From a eurorack perspective it feels like another giant knob and sequencer, but with lots of options and big buttons it's much more deliberate and inviting than the more experimental feel that you often get with individual modules.