The question is not clear. And you give us no info on the capabilities of your "main keyboard".

You set the octave of the MOX6 by editing the MOX6. But it is not clear if you want the MOX6 to sound an octave different from normal, or you want the MOX6 to also transmit an octave different.

In general, the control over your slave modules/synths is programmed at whatever you are using as your main keyboard.

Here's what I mean, if the MOX6 were your main keyboard I would instruct you to set the TRANSPOSE parameter to change what the MOX6 sends out via MIDI, or adjust the NOTE SHIFT parameter if you only wanted to change how the MOX6 sounds without changing how it transmits.

Hi, thanks for taking the time to reply. I really appreciate it.

I use a Roland RD-700SX as my mother keyboard. From this I change the patches on the MOX6 which sets up both keyboards with the correct sounds. I often find that I would like the MOX6 to be in a different octave when I play it and it would be really useful to have it change octave as well as the voice. I can set a transpose parameter on the RD-700 (via its TRA setting in its EXTERNAL ZONE) but this only changes the MOX6 when played via the RD-700. I wonder if there is some midi instruction that can be sent to set the octave on the MOX6 so that I don't need to press the OCTAVE button on the MOX6 as I am playing.

Sorry if that sounds a bit confusing!
Thanks again for your help !

F0 43 10 7F 1C 00 00 00 08 dd F7

where "dd" is the Data Byte (3D-43)
3D = down 3 octaves
3E = down 2 octaves
3F = down 1 octave
40 = normal
41 = up 1 octave
42 = up 2 octaves
43 = up 3 octaves