I prefer to use oxford comma where I'm writing a long list of items, and those items contain sets within themselves.

For instance, while listing computer brands that include Dell, HP, Acer, Toshiba, Alienware, Compaq and ASUS, I wouldn't use the oxford comma. But if I'm listing out speaker brands like Bose, Klipsch, JBL, Bang and Olufsen, and Altec Lansing, I think oxford comma helps to clarify the meaning intended.