As confusedbuffalo said, access is mostly tagged at the level of paths or entrance nodes.
I suppose a tourism=attraction tag can imply access for tourists. I would not add that tag to a college if it were always closed to visitors. (However, an attraction could be significant even if viewed only from outside, e.g. the Radcliffe Camera.)

Moreover, Brasenose *is* open to visitors, usually. https://www.bnc.ox.ac.uk/about-brasenose/for-visitors

Colleges are educational institutions first. Their opening hours and practices with tourists can be idiosyncratic. I don't think there is a way of consistently tagging that (they would be access=private mostly, with some complicated conditional access possibly).
