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In the example below, the sentence appears confusing because of its length and the double use of the connector "and".
The study of oral health examines the causes of oral diseases and the impact of oral care habits and behaviors.
What is the best way to clarify a sentence with multiple connectors like "and" in a horizontal list?
Please notice, Oxford comma should be not used.
A comma here, "...oral diseases, and the impact of..." makes the meaning unambiguous.
There are two lists in the example sentence, one nested within the other:
The study of oral health examines {the causes of oral diseases} and {the impact of [oral care habits] and [behaviors]}.
The two items in braces, { and }, form one list. The two items in square brackets, [ and ], are a different list. This second inner list is nested inside the second item of the outer list.
Adding a comma where I've recommended makes it clear what's the outer list and what's the inner list, so there's no ambiguity.
As this is two lists of two items, there's no possibility to use an Oxford comma:
Wikipedia:
"In English-language punctuation, ... an Oxford comma ... is a comma placed immediately after the penultimate term (i.e., before the coordinating conjunction, such as and or or) in a series of three or more terms. (my emphasis added)
As an Oxford comma requires a list of three or more items, this comma cannot be considered an Oxford comma. It's there only for clarity.
