According to the Admissions Tutors I know, interviews rarely change the pre-interview candidate ranking or pre-interview overall assessment.
As an Oxford Admissions Tutor I can say this is absolutely not true in my experience (19 years' of it). Interview marks and pre-interview marks are correlated, but certainly not the same. And, if anything, the interview marks probably drive the ranking more than the pre-interview marks (though this does vary by subject to some extent). As someone else said, there would be no point interviewing if we didn't use the information to change the ranking. We don't do it for fun!
I would also say that the whole process of reallocation/pooling/second interviews etc has changed significantly over time, and does vary between subjects, so you should be wary of relying too much on past (often old) experience. But broadly speaking, some subjects (mainly sciences) set up interviews across two interviews from the start. For those that don't, second interviews tend to be given to candidates considered by their first College to be likely to be good enough for an Oxford place, but it usually means there may not be space at the first College for them.