Train/ oyster card confusion

[laanndaaannanny]

This might be a stupid question but I've just got an oyster card and I want to get a train/tubes on a longish journey. Normally i'd book this in advance on trainline.com but can I just swipe my card as I'm going instead, is this cheaper or is it cheaper to book it online first.

Oyster can be used on trains only in greater London, so you'll probably need a train ticket from the edge of your oyster geographical area (if you have a travelcard) or for your whole journey (if you have pay-as-you-go).

Oh okay thank you that makes more senses now