Except the Oxford comma PREVENTS confusion, and thus should be the rule.
General rule: don’t use the comma. If the sentence is ambiguous to the reader in a specific case, then either: add a comma, or rewrite your sentence to be less confusing.
For example, “Apples, oranges and pears” the “and” has the about the same count as the pause after “Apples”; pausing before the “and” feels like a stutter.
"We're having hot dogs, hamburgers, and peanut butter and jelly.
Especially in a medical setting where the aesthetics of a sentence are not close to as important as clarity.