Only just seen this thread. How were your visits @HattyBee ? We are a A420 family too so had to go through the same process.
IMHO the key thing is to decide where you want your DC to go for senior school and work back from there. (I know that feels ridiculous when your children are six or seven and who knows what they will be like at 13!)
If you think you want them to go to a local coed day school like Cokethorpe or Abingdon, then that takes you in the direction of the prep departments of those schools. Admission to the senior schools isn’t automatic, but it is a lot easier and you will know a lot of the children and parents already. St Hugh’s has also just announced it is opening its own senior school so its prep will be very keen to keep children there rather than preparing them to go elsewhere at 11 or 13 as has been the case until now.
If you don’t know yet/aren’t willing to soft-commit to an Abingdon or a Cokethorpe or St Hugh’s for the next 11 years, then a third option is Chandlings. Chandlings is a day only coed and sends children to all the Oxford and Abingdon day schools. Because they don’t have an attached senior school (it recently became part of the Radley Schools Group, but nobody ever goes from Chandlings to Radley) it is a good choice if you know you want 11+ day but aren’t sure which yet.
If you think there’s a chance you might want any of them to go to one of the big public schools to board in Y9 (Radley, Eton, Marlborough etc) then you need to look elsewhere - to the likes of Cothill which offers day as well as boarding, or further afield to Summerfields (boys only) or the Dragon in Oxford, or to Pinewood which is a big Marlborough feeder. All those four do very strong 13+ prep and send children to all the top boarding schools. Abingdon and Cokethorpe preps, St Hugh’s and Chandlings are not set up to send pupils to those schools, and lots of the year groups leave (or move up) after Y6 rather than Y8. Also, while St Hugh’s still does a bit of flexi boarding, it’s more like a babysitting service, not really the experience of boarding that is useful when applying (or considering whether to apply) for a senior full boarding school.
PS Am a bit mystified as to the comment by someone earlier that “Cothill/Chandlings has just been taken over by Radley so days are numbered”. Cothill and Chandlings are completely different schools - one is mostly boarding and goes to 13, one is 100% day and goes to 11 - and send children to totally different destinations. It’s true that the charity that owns Radley has just taken over the charity that owns Cothill and Chandlings but it’s hard to see why it would have bothered doing that if the plan was to shut them down. Radley always has a big intake from Cothill so it would be really counterproductive!