As far as I am aware, London Overground remains part of National Rail
and normal NR fares will apply beyond Harrow & Wealdstone. The fact
that an Oyster Card can be used to pay for those fares by PAYG does
not mean that LU fares will be charged. After all, London Buses accept
Oyster PAYG but have different fares to LU.

Nor does it mean that there will be LU-style differential fares for
using Oyster PAYG as opposed to paper tickets. National Rail has not
signed up to that concept.

Were reduced fares to be brought in south of Watford Junction, under
the fares basket rules the affected franchisees would have the ability
to increase other fares by more than average to make up the
difference. So customers south of Watford would pay less whilst those
living north would suffer.