The app will only work with contactless cards and/or Oyster cards; not Oyster photocards or other smartcards. You can only use the app if your Oyster card shows a 'D' symbol in the bottom left of the reverse side, as shown in the below image.
Is it cheaper to pay with an Oyster?
You can add money to an Oyster card or use contactless (card or device) straight away. You only pay for the journeys you make and it's cheaper than buying a paper single or return ticket (train companies may offer special deals on some journeys).