I’m not sure if I follow you on this one? Reservations are not linked to the pass what so ever.
The pass = your ticket. Reservations (for free and only optional on domestic UK routes) are not tickets, it just guarantees you a seat.
So for every train you take, with reservations, you need to add the journey to your pass and activate a travel day to travel.
You can’t travel with just a reservation, since you’ll be considered as a fare dodger.
London-Oxford isn’t that far, so you might want to buy a normal ticket. If you take the slower train it’s 8-12 Quid one way bought in advance, 35 quid for the faster train one way (only 30 min quicker than the slow train, not worth that big price difference).
Anyway this relatively short journey is not worth the cost of a pass day if it’s your only journey of the day. Buy your ticket (for the slower service) a couple of days in advance on www.GWR.com or in a GWR- station.