...and so much more useful! Why *can't* you load a ODTC onto oyster?
Surely its not technically difficult?

Because it wouldn't be a true ODTC, because you couldn't use it on
National Rail.

And why not? The National Rail ticket gates are perfectly capable of
accepting period Travelcards on Oyster and Off Peak ODTC on paper tickets so
it doesn't seem a leap of technology to accept Off Peak ODTC on Oyster if
treated in much the same way as a period travelcard.

I suspect the issue is that for sensible implementation it would indeed have
to occupy one of the slots for a period travelcard (there are two I believe)
and that could cause confusion for those who prefer to buy multiple ODTC in
advance. Also if purchasing online you'd have to go through a tube station
gate to get it loaded which is probably extra inconvenient for one day
travelcard users. I doubt capping was expected to be so delayed either so
it was really only an issue for National Rail ODTC users.