While I’ve been sampling the delights of the Paris transport system over the weekend, TfL have removed the ability to pay using cash on their buses. You can use Oyster, of course, or contactless payment cards (CPC), or a paper travelcard. The only thing to note is that at present a CPC will overcharge you if you make four or more bus journeys in one day. Proper capping will be introduced later this year, along with the ability to use CPCs on rail services.
The other concession that has been introduced recently is the concept of one more journey. If your Oyster card has run out of credit you can make one more bus journey before you need to top up. This is to avoid stranding passengers making late night journeys when most ticket stops are closed.