Most chinese words about the name of foreign places are mapped phonetically, such as 华沙, 纽约 or 首尔, I believe. But in some cases it is from the historical incident, such as 旧金山 (I don't know how many are from this rule). Also, Japanese places are directly from the equivalent Japanese characters, such as 东京 or 北海道.
But why is Oxford called 牛津?
Also, in general is there any rule regarding that the word should be mapped phonetically or not?
The related question focuses on who decides the names but I would like to ask how they are decided.