There is a Senior Oyster card but it is strictly limited to London residents.
If your mom has a Senior Railcard (£30 cost), she can use it to get 1/3 discount on off-peak Tube travel using a regular Oyster card:
But the limitation to off-peak travel reduces the value of this benefit. And since you are driving once you leave London, she will have little other use for that £30 Railcard.
Also, it does not discount bus fares, and I believe it is your intent to use mostly buses.
But apart from transport, she may well find Senior "concession" prices at many London museums and other attractions. We benefited from these last May. There was not a U.K. Residence requirement; we just asked for the Concession price. And much to my dismay, I was not asked for proof of age.