Voltage level difference when using an external power supply
In the future, draw diagrams with logical flow left to right.  It was confusing at first since your DC-DC booster and 5 V buck regulator are shown backwards, and in/out is not labeled.

The basic problem seems to be that this Oric thing's power supply reference and signal ground are not the same.  The simplest way to solve this within your diagram is to provide the Oric with its own isolated supply.  Then it can float arbitrarily, which allows you to pick any one of its lines and tie that to your ground.  You would pick the signal ground on the expansion port.

You could just get a separate 7.5 V power supply for the Oric and be done with it.  If you really need to create the 7.5 V from the 5 V supply for some reason, then that converter needs to provide isolation.  You could do this with a flyback converter, for example, since those are inherently isolated via the transformer.  There are also many off the shelf DC-DC isolated modules available.  7.5 V is a rather unusual output voltage, buy maybe you can find one if you look around.