PAYG on the Overground (and on other NR lines where Oyster is
accepted) is treated the same as PAYG on the tube, and hence a journey
that involves both is only considered as one journey. I think this is
true for the DLR as well.

Buses are different. $deity knows why, although my guess would be
technical implementation reasons rather than an ideological dislike of
mixed-mode bus-rail journey.

I looked this up on the TfL website and it seems to be true. I have
to admit I didn't realise. Probably because I have only got my Oyster
since moving out of London, and my trips to London are now generally
confined more to the centre for both business and pleasure - so I am
less likely to use Overground services. On top of which there have
been a lot of changes in the last year or so. I know in the past I
have avoided using DLR because I thought I would be charged extra, but
then the stations are close together and I often like to walk anyway.

I suppose buses are different because there is no "end" checkout. But
they could easily bring in a rule whereby if you used a bus within,
say 30 minutes of the start of a recently completed Tube journey, you
wouldn't be charged.