An abridged explanation of the rule is thus: Omit the Oxford comma in a simple series.
Example: I went to the store to buy bread, milk and cheese.
Previously, the rule stated that an Oxford Comma was used if the final words in a simple series were a group (and included a conjunction such as “nor,” ”or,” “but,” “and,” etc.) or if you were discussing distinct groupings.
Example 1: I went to the store to buy bread, milk, and Swiss and cheddar cheese.
Example 2: I went to the store to buy milk, soda and juice, and cake, cookies and chips.