Radon fan

I would keep it away from an OAK inlet if you are using one....Perhaps if you are worried about Radon it would be better to use household air for combustion anyways to help remove it. As far as clearance to just the stove exhaust I doubt it matters....This is all assuming the Radon fan is set up to remove household air, not to bring in fresh air!!

Radon fan is pushing out, not pulling in.

Radon is heavier than air so it will tend to migrate downwards so a fresh air vent to a stove would best be mounted above it. Arguably any radon that gets reintroduced goes into the combustion chamber and out the exhaust so it doesn't get back into the living space.

I

Radon fan is only pushing when it's 'on'

Well, I doubt it should matter since the radon gas, >5ppm, will go in your OAK, and then out your exhaust vent. BTW, most radon vents, vent above the roofline, like your stove exhaust, nowhere near an intake.

The radon usually vent above your roof line no worries about sucking it in. I think your only concern would be its proximity to the exhaust because of heat. Id recommend you go with your vent manufacturer or stove manufacturers spec. I think 3" is somewhat usual.

Radon is an interesting point to consider. If you have a radon mitigation system, which draws air from under the foundation and vents to outside, I would highly suggest an outside air kit. Don't really want to compete with the Radon system by drawing that air into the house.

While my Radon system is no where near my stove, it has the fan outside in a pvc box with pvc pipe. That's the aspect that would drive the spacing in my case, which I would take as the same as to vinyl siding.

Good luck,
- Jeff