I rarely use them south of Reading, but as someone who travels Reading to Oxford frequently, the majority of people are boarding at these two stations - trains rarely pull into Reading without empty seats, but often leave Reading with no empty seats, but rarely "full".
The issue then comes when reaching Oxford, where usually there are tonnes of people on the platform and not many alight - I think Oxford station has seen mass usage increase in the past 10-15 years, and both the London trains and the XC trains were not equipped to deal with this.
Therefore, I don't think pick up only at Oxford would work either, since the issue is not Reading to Oxford journeys, the issue is journeys north of Oxford, which makes much more sense since there are very few feasible alternatives for this journey.
The answer is more capacity and making the Reading - York's (I don't think sending them to Newcastle is necessary) hourly.