wrote

The things where a travelcard is bundled into a day return (which
Chiltern have done for a while, but Virgin now seem to do as well) are
marginal: they're six zone, which is good if you're going to use it
but less good value if you aren't. And I'm never entirely sure
(perhaps someone could comment) on if they include buses.

If your National Rail journey is via a central London terminus the off-peak
ODTC costs about the same as a CDR plus two Zone 1 tube journeys on Oyster.
So if your travel in London is more than this, and especiall;y if it
includes any other National Rail journeys, for which Oyster P&G is not
valid, then the ODTC is better value. However, if you change from National
Rail to TfL at a suburban station (Upminster, Barking, Ealing Broadway, etc)
and then only use Oyster on TfL within London, then a CDR plus Oyster is the
answer.

Peter