Perhaps somebody will have better ideas than mine, but this one feels like it should be cheaper than it is.
Option 1
An outboundary off-peak day Travelcard from Denham is £13 (would be £12.35 but there's a £13 minimum fare) and is valid for unlimited travel in Zones 1-6. You then just need extension tickets to get to Hatfield and back, and to Oxford.
An off-peak day return from Boundary Zone 6 to Hatfield is £6.90 (no discount on this because of the Network Railcard minimum fare).
An off-peak day single from West Drayton to Oxford is £15.95 with the Network Railcard. The West Drayton ticket is fractionally cheaper than the Boundary Zone 6 to Oxford ticket. It is only valid on trains passing through West Drayton (so anything going from Paddington via Reading). There's no need for the train to call at West Drayton because your Travelcard is a daily zonal ticket. If you want the option to go via High Wycombe, however, get the Boundary Zone 6 ticket instead. So that's £35.85 in total.
Option 2
I did wonder about a simple day return ticket from Oxford to Hatfield at £33.35, but by my reckoning that works out more expensive than option 1, not least because Barons Court is not in zone 1, nor is it on a plausible route between any pair of National Rail stations appropriate for the through journey you will ultimately be making, so I'm not at all sure you would be able to end your cross-London transfer short at Barons Court. I suppose if you said you had left the National Rail network at Ealing Broadway (which would be a lie if you were coming from Denham), then it might look reasonable for you to take the Eastbound District Line by way of Barons Court towards, for example, Blackfriars for a Northbound Thameslink service to Hatfield, in which case you could hypothetically end your transfer short at Barons Court, but I think that's stretching a point. Consequently, I suspect you would have to pay separately for the tube between Marylebone and Barons Court and between Barons Court and wherever you choose to pick up the train to Hatfield. Even on the heroic assumption that you only had to pay one tube fare rather than two, this would still cost fractionally more than option 1 and would give you less flexibility.
Option 3
Given that Project Oval is supposed to be the answer and the Transport Secretary is trumpeting her best fare guarantee, I did look at the price if you were to use Contactless wherever possible. If you did Denham to Barons Court, Barons Court to Hatfield and Hatfield to Reading on Contactless, changed trains and tapped out at Reading, then bought a single from Reading to Oxford, by my reckoning that would cost £8.05 more than option 1. I'm unaware of any caps that would serve to reduce the cost of the single journeys in this itinerary.
