I briefly commuted from Ealing to Wycombe by car a couple of years
ago. To get from my house (2 miles south of the A40) to the A40 took
about the same amount of time as it did from joining the A40 to
getting to my work in central Wycombe, and this was leaving after 9am,
using the Argyle Rd/A40 junction. I was returning late evening so
obviously quicker, but not a fair comparison on the return. Depends
where you live in Ealing I guess but generally all main roads in the
area are an nightmare in the mornings. Have you considered the train?
There are direct trains from Ealing Broadway to Oxford, and also from
West Ealing and Hanwell with a change; it's not necessary to go into
London first. I think the fast trains Ealing Bdy-Oxford take a little
over an hour; more like 1h20 if you have to change/catch a slower one.
Don't know how busy they are etc but as you'd be going out of town I
would expect them to be significantly less busy than in the other
direction; you'd probably get a seat, and could use the time
productively, etc.