Can you post a picture of where you are getting interference? I did not do anything special. My brake carriers sit about 1/2" below the chainstays, so there is plenty of clearance in all directions. I am running a HED Plus disc, so that is pretty wide too.

  

The images are here : https://imgur.com/a/lFIxKeD
I gave up with the omega X and reused my first gen Omega. But you can still the same issue here. The pads are way too close to the rim of my FLO Disc.

Need to shave off the brake pads until there is little left. It's common on lots of bikes when wide rims are used.

Aaaah, never mind. I forgot that this was a P5 thread. I was showing my IA16 undermount to help the OP sort out his P5 problem. What rruff said... you will probably need to shave your brake pads for a P5.

Hah xD. That would do it. I was looking at your pictures thinking, what am I doing wrong? theres no way the arms will fit so neatly in the chain stays.
I dont think its a spacing issue with the brake pads themselves, while yes that would be one way to solve it, I think the issue is that the shape of the pad holder is too wide at the back end (the left edge in the picture below) So even though the brake will engage, there isnt enough room for the brake to actually release backwards.

I tried mounting an omega x on the bb of my P5 with no success. Had the same problem as you with the brake pads hitting the frame. Also had a problem with the cable kinking. Gave up and went back to the TRP mechanical under the bb. Using omega x on the front though.

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As rruff said, start with using thin pads (shave and/or cut them down - or use some worn ones). Next thing you can do is space the brake a bit away from the frame. I also used some spacers/washer that allows you to angle the brake a bit so I angle it a bit away from the frame to get a little extra clearance for the brake arms. It is definitely a tight fit.

It's currently installed using Jagwire Elite Link brake cable and not using the brake cable holder under the BB to give the cable less bend. I've not ridden outdoor with it yet, but it seems to work OK with the brake releasing without problems and the brake levers not being mushy.

Yes, I see your problem. The pad holder adds additional thickness. If you want it to work and don't want to shave your pads, I'd cut off the holder so it fits.