Why is Oxford called 牛津?
This is a rare example of Chinese translation for foreign cities or the similar, there's no rule to govern this type. In short it translates literally the meaning of Oxford:
Ox: 公牛， 牛
Ford: ferry, 津,水渡也。——《说文》
Hence, Oxford == 牛津

The meaning of ferry in 津 is not seen often in modern Chinese, here are a few examples:
天津， 天子之津渡。
城阙辅三秦 风烟望五津  ----送杜少府之任蜀川   王勃
无人问津 ---成语

Also two examples of naming:
Little Rock, 小石城
Springfield， 春田市， 斯普林菲尔德 also work.