The Expression Assign section of the Moog One has seven buttons that can be used to instantly map some of the most commonly used modulation sources, including the x- and y- parameters of the X/Y Pad.
However there are also numerous other modulation sources that do not have dedicated buttons, including the X/Y Pad’s pressure. To access these, simply select the MOD button on the One’s center panel. This gives you access to every modulation lane in use on the currently selected synth layer.
Use the first soft knob to scroll through the various sources until you find “Pad Pressure.” From there you can set the source percentage, controller, transform, and the destination you want to control with the pressure.