To stop your Korg opsix from automatically starting its internal sequencer when receiving play/start messages from a DAW or external gear, disable Rx Transport in the global settings. [1]
How to turn off Rx Transport
1. Hold down the SHIFT button.
2. Press the EFFECT/GLOBAL button.
3. Turn DATA ENTRY A to select MIDI.
4. Turn DATA ENTRY B to select Rx Transport.
5. Turn DATA ENTRY C to select Off. [1]
The opsix will now ignore external MIDI start and stop commands. Its sequencer will only start when you press the physical play button on the panel. [1]