Not sure about that last bit regarding Oyster but I could be wrong. AFAIK the Senior Railcard is for National Rail services only and can’t be linked to an Oyster card. You can still use the Senior Railcard in London to get the 30% discount on overground train fares but not if you use an Oyster and not on other transport (tube, bus) irrespective of whether you use an Oyster or other payment method. The 60+ Oyster card is a different animal: it's only available to people with a residential address in a London borough, and it permits free travel.
Correction: Apologies
@VPS, you’re right and the Senior Railcard
can be linked to an Oyster. I have just looked further into this and, according to the TfL site, it can only be done by first registering your Oyster card with TfL and then approaching a staff member at a London underground or overground station. And the 30% off fares for which you can pay by Oyster only applies at off-peak times. Wish I had known this when I was in London recently….