Self-catering might be difficult for you, because generally the older colleges will put you in their main, old buildings in the college in one or two of your years, and those buildings don't always have student kitchens. (Magdalen does: see guide
here). At Lincoln where I went, 1st year undergrads didn't have access to kitchens, but 2nd and 4th years generally did.
Accommodation varies a lot from college to college; some offer college accommodation to undergrads throughout their course and some don't. But many of the older ones will not offer you access to a kitchen all through your studies. If this is very important to you, I would draw up a shortlist of 5-6 and check what facilities they have (email the JCR committee and ask if the college/JCR website isn't informative enough).
For green spaces, I guess Magdalen does pretty well with its deer park! Christ Church ofc has the meadow, but that's open to the public so you can go there no matter what college you're at. Merton has quite big green spaces at its main site as well. Somerville's gardens are fairly big. However, Oxford has plenty of green spaces. A lot of people love to go to Port Meadow, which is by the river, and then there's the University Parks and South Park. And then there's Christ Church Meadow. So no matter which college you're at, you will always have big green spaces to go to if you don't mind walking a bit/getting on a bike. These places are all within 20 minutes' bike ride of the city centre.
As far as 'friendly environment' goes, I haven't experienced many other colleges apart from my own, so I'm not the best person to say, but I am inclined to believe that every college is capable of being friendly to people who fit in there; I don't think there is such a thing as friendly and unfriendly colleges. Bear in mind, the student intake changes every year, and every year there is a different JCR committee in charge of looking after undergraduates. They set a lot of the tone, as they organise a lot of events. A good or bad JCR committee makes a big difference to how sociable and happy a college is. And that's not something you can predict or plan for.
So the question is, where would you fit in? Some colleges are bigger than others; the advantage of being bigger is you're more likely to find like minded people or people with the same interests as you. Some have more international students than others (I suspect that the famous colleges like Magdalen, Balliol and Christ Church fall into this category). And some colleges have a noticeable political alignment. Christ Church is traditionally right wing, although being big, I expect it's more diverse than that would make you think. Oriel (sometimes called 'Toriel') is definitely right wing. Wadham is pretty left wing and has a reputation for being woke. I would also look into whether any college has a particular club/society that you'd like to join. A lot of people (not just athletes) get into rowing at Oxford, not necessarily because they are mad about sport but because it seems quite addictive for some people, and also, you get to be part of a close-knit team. You'll train together, have breakfast after morning outings together and you'll party together. That's a great way to bond with a group of people (if you can tolerate getting up early for training, including in winter).
Depending on which country you're from (which country are you from?) you may well find a university society for people from the same country where you can hang out with fellow nationals. That is useful if you feel a bit isolated at college.
Hope that helps! Other people may have slightly less vague advice...