Travelling partially on oyster card, partially on a train ticket for the same journey to save money?

If you're using Oyster Pay as you Go, the terms and conditions require you to touch in and out at the ends of your journeys. While you might have luck getting Oyster's helpline to refund your journeys, this is by no means a guarantee, and so you shouldn't rely on getting this refund. A validated Oyster Pay as you Go ticket I suspect also won't be accepted by many guards or ticket inspectors on a train that doesn't stop at any Oyster stations.

If you can get off the train and touch out at the station, you can do that. You might also want to look at split ticketing - perhaps two paper tickets would be cheaper for you?

You can buy the ticket you need from some ticket vending machines (look for a "tickets from other stations" option - some have them, some don't), all ticket offices, or online from any train operating company's website (and pick it up from a machine at your origin station). I doubt most staff would bat an eyelid at a request to buy such a ticket. You don't need to buy it at the station you tap out at.

If, on the other hand, you have a Travelcard season ticket loaded onto your Oyster card, there is no need to tap out. Just stay on the train. For tickets for such circumstances, you can buy a ticket from "Boundary Zone N" (where N is the last zone of validity for your travelcard season ticket); you'll probably have to do this at a ticket office as few machines and few websites offer them. Note that none of this applies with pay as you go!

I do this quite often (I think approximately one return every three weeks) when I interchange between National Rail and London commuter trains because Clapham Junction doesn't have Oyster Readers on the platforms. (Although I am travelling one stop on Oyster and 150 miles on National Rail.)

I am charged the maximum fare, and I call up Oyster when I have a few minutes at the weekend and claim back the £13.00 or whatever it is. The telephone operator sees the incomplete journeys and I tell them the proper completion of it. They offer to put it back on my Oyster card or refund it back to my bank. It's a bit of a hassle, but the telephone is answered quickly and it is fairly painless. In principle they are entitled to refuse, but this has not happened yet.

I also tend to have a few incomplete journeys from tube barriers not properly registering my card or from when I've been advised by station staff to keep walking because of severe overcrowding. My last reclaim actually came to over £50, and this was dealt with quite efficiently with no hint of resistance.

In theory this is not the proper way to travel and London Transport is entitled to refuse to make the refund. I think if you did this on a daily basis it would not work so well. (Though, if you do this on a daily basis, you can buy a season ticket, which does not require you to touch in or out.)