Call it what you want: the Oxford/Harvard/Series/Serial Comma. Let's use it.

The MLA style guide (and other styles, including CMS [Chicago Manual of Style] and APA [American Psychological Association]) require the use of this comma because it reduces the ambiguity of our writing. Although several other style authorities (including the AP stylebook [Associated Press stylebook] and the CP [Canadian Press]) recommend writers avoid using this comma, we should use this comma consistently in our projects.

Many of the conflicting guidelines between writing styles are rooted in the differences between British and American styles. Most (but not all) of the authorities recommending against the use of this comma are non-American style guides (including British and Canadian guidelines). While we won't be writing for an exclusively American audience, we should follow our most widely accepted, American guidelines (some British style guides, including the Oxford Style Manual, also require the use of this comma).

While the introduction of the serial comma can introduce ambiguity, this is seldom the case. Since including this comma in my writing, I have felt more confident in my writing and have rarely encountered a situation where the omission of the serial comma would have added clarity or reduced ambiguity.