Paul Terry wrote:

I've never seen a good reason why a (pre-purchased) ODTC on Oyster
shouldn't be acceptable to the TOCs (pre-pay is a more complicated
issue, of course).

AIUI, it seems to be because for some reason you can't load one day
travelcards onto Oyster cards, rather than that the TOCs won't accept
one day travelcards on Oyster.

Patrick