Q. What is the serial comma in APA Style?

The serial comma, also known as an Oxford comma, appears before the last item in a list of three or more items (American Psychological Association [APA], 2010, p. 88; APA, 2020, p. 155). Both the 6th and 7th editions of the Publication Manual of the American Psychological Association require authors to include serial commas; see page 88 in the 6th edition manual (APA, 2010) and page 155 in the 7th edition manual (APA, 2020) for information.

Example:

Correct: The stripes on the book are black, orange, and yellow.
Incorrect: The stripes on the book are black, orange and yellow.

References

American Psychological Association. (2010). Publication manual of the American Psychological Association (6th ed.). Washington, DC: Author. 

American Psychological Association. (2020). Publication manual of the American Psychological Association (7th ed.). https://doi.org/10.1037/0000165-000