The 15 min city idea sounds great, if it is planned that way. Trying to impose it on an existing city without huge amounts of additional funding just simply isn't going to work.
When I lived abroad, I lived in a rapidly expanding town. They were building doctor surgeries, schools, dentists offices, grocery stores, affordable housing (for the employees of the schools, doctors surgeries etc) as fast as they could as well as sorting out the infrastructure (roads, drainage, sewerage, public transport etc). Here they're just saying 'you can't drive on these certain routes and have to go massively out of your way on the ring road / a34 instead' and doing sod all else.
As usual in the UK it will affect the poorest and most vulnerable the most and make their already very difficult lives even more difficult whilst the richer residents of Oxford get what they want.
The only plus is that jobs in retail, care etc have so many vacancies I think those who originally moved into commuting communities to do jobs in Oxford will find it easy to find jobs closer to home and just simply stop going into the city at all.