If one graduate at a developing country which lack highly ranked professors, how much weight should be expected to be given from Oxford University to recommendation letters for a Dphil? Do they weight it as much as letters of students with famous letters from famous professors? Are students with letters from famous professors preferred or they just access the content?
I doubt there are any statistics to support this, so this is my opinion plus some limited experience in selecting applicants to graduate programs. A famous professor's letter is likely to have somewhat more weight, but not purely because they are famous; more because the fact that they are famous implies that they have more experience judging people. If someone who has mentored two students says "This is the best student I have ever had", their ranking has less weight than the same claim from someone who has mentored two hundred.
On the other hand, a strong recommendation from someone who knows the applicant well will have much more weight than a vague and impersonal recommendation from someone who clearly doesn't know the person as an individual. Remember that committees already have much information about the applicant as a student -- grades, C.V., courses taken, extracurricular activities should all be present in the application elsewhere. A recommendation that merely says, for example, "This student received a good grade in my course" is useless; we already can see that, it's redundant information.
A useful recommendation letter adds new information that the committee can't get elsewhere. A student may have received an "A" because they are slow, but diligent; or they may be brilliant but lazy. The student has published a paper -- Did they perform a technical but unimaginative role, or did they conceive of the idea and push it to a conclusion? The outcome will look the same on the other documentation, but the recommendation letter can tell the committee what actually happened. Similarly, committees look to recommendation letters to try to understand the applicants' personalities. Are they cooperative, argumentative, shy, stubborn, agreeable? There is wide latitude for personalities in graduate studies, but there are certainly some people who are unlikely to do well because of their personality, and a recommendation letter can help flag those who are likely to be great fits.
If that new information comes from a famous person who has had the opportunity to compare to many other students, so much the better. If it comes from an obscure person, it is still far more useful than a mere recitation of the same information that's already present in the CV; and it's far more useful than unhelpful information, no matter how famous the person.
(I have seen cases where applicants included personal recommendations from senators and in one case the US president. These letters not only did not help, they reduced the ranking of the applicants, because they were inappropriate and reflected poorly on the applicants' judgement and understanding of the field.)
Well from my personal experience, I worked in an office where we were sending students to world top ten universities from Pakistan. We sent some to Stanford USA, and Oxford UK beside others. What I saw was that indeed weight is given if you are from a reputable university or not in a bad one at least. And that most of the weight is given to grades but most important is the research proposal that you write and your personal statement.
Also, a letter of recommendation not only states about you, but also the writing expresses the professor himself, how good he writes it.
Then if there is some research that has to be related to a developing country, then they will prefer you, especially if it's related to your country or like wise.
I myself got a scholarship in a good university under top 100, though lot of my friends with excellent grades didn't succeed. So keep trying is the key.