In message , Demiurge writes

From September 11th I'll be travelling from East Croydon to Brixton (train
from E.Croydon to Victoria, then tube to Brixton), peak times, on Mondays,
Wednesdays and Thursdays. I'm planning to buy a one-day travel card each day
(£10.40 per day). Would Oyster be of any use to me, i.e. reduce my costs at
all? I've never used it before.

You cannot put one-day travelcards on Oyster, nor can you use Oyster
Pay-as-you-Go on most of National Rail. So, I haven't done the sums for
you particular route, but it is usually the case that Oyster is more
expensive than a paper travelcard for occasional journeys that involve
National Rail.
--
Paul Terry