The "serial comma" - sometimes called the "Oxford comma" or the "Harvard comma"- comes immediately before a conjunction that separates the last of three or more elements in a series. For example, consider the trio "ready, willing, and able." Consider too "win, lose, or draw." The serial comma is the one immediately before the "and" or the "or." In statutes or private arrangements, a comma's presence (or, as in O'Connor, its absence) may hold high stakes for litigants.
Douglas E. Abrams,
The Pesky Serial Comma, 73 Journal of the Missouri Bar 212
Available at: https://scholarship.law.missouri.edu/facpubs/764