I would not use CC numbers to change presets. It’s better to use PC numbers, as it states in the Opus manual on section 5. MIDI. The way things would work is you load a preset on the H90, let’s say preset 10, then the H90 sends PC#10 to the Opus. Ideally, you would Program presets on the Opus to match up with the loaded H90 preset.
If you prefer to jump around, for example, have Preset 10 loaded on the H90 but use preset 97 on the Opus, it’s better to use a MIDI controller that transmits to each pedal on its own MIDI channel, since the H90 only sends the PC# for the Program that it’s on.
With regards to using the aux switch, in the MIDI section of the System menu you have the “MIDI – Global Control” where you can assign a switch to control functions such as increment, increment + load, decrement, decrement+load, and simply load. So as you press the aux switches it will change Programs on the H90, which in turn will change presets on the Opus. You can’t program an aux switch to go to a specific Program, if that’s what you’re trying to do.
The other option is programming the aux switches to transmit a CC# to the Opus, but you have to program that CC# in the “Pedal – Transmit” section of the MIDI menu. So, if you say CC#68 selects preset in the Opus, then you can map a switch to always send CC#68. But again, it’s not typical for CC messages to be used to recall specific presets. That’s what PC#s are for.