Aussiefolk

We are spending a week in Oxford in May. We plan to venture in to London on a couple of those days and need advice in regard to using the train. In the past we have used an Oyster card if I recall.
It is the first visit to London for my travel partner. Is a ‘hop on hop off’ bus still a reasonable way to explore the major attractions? (I know we’d only be scratching the surface but, sadly, we’re unable to spend more time in London)
Also, is there a London Pass or similar these days? Visits to London in the past were brilliant using this pass.
thanks in anticipation.