Here’s a checklist for MIDI sequencing from the OT:
MIDI connections
Connect the MIDI Out port of the OT to the MIDI In port of the target synth using a working MIDI cable.
Octatrack MIDI track settings
In the OT’s MIDI mode, select the MIDI track that you will use for sequencing. In the NOTE SETUP page, set the MIDI channel number (“CHAN”) on which the Note messages will be transmitted to the synth using knob A. Remember to press either knob A or the [ENTER/YES] button to activate the channel setting.
On the synth
Ensure that its MIDI channel number matches the number that you set on the OT.
Now, if you enter trigs on the OT’s MIDI track, MIDI note messages should be sent to the synth. Let us know whether this works. If it doesn’t, we can do more troubleshooting.
Edit to ask: are you instead trying to use the OT’s audio tracks to sequence the external synth(s)?