You have to take two bus journeys to get to the Kassam from Abingdon, I know that as I used to do it. And I think the same can be said for almost any town or village in Oxfordshire, as you need to get one bus to the city centre and then another out to BBL.
I think Stratfield Brake is easier to get to from virtually anywhere else in the county than the Kassam is due to Oxford Parkway being next door, except east Oxford. Although granted, a lot of people and OUFC fans do live in east Oxford which isn't ideal, but then as I've said it's been a lot more difficult for anyone outside of east Oxford while we've been at the Kassam.
Also, you could always get a bus to the city centre and then a 5 minute train, which would likely be quicker than taking another bus.