This could get ugly, because there seems to be no middle ground with the Oxford comma debate.
For myself, I don't believe in it--although there are exceptions.
One use of the comma is to indicate something has been left out. For example, you can replace "then" in an "if . . . then . . ." statement with a comma. Such as, "If the front door is locked then go around to the back" can also be stated as, "If the front door is locked, go around to the back." "Then" has been left out and a comma takes its place.
So the way that kids will list items might sound like this: "I got in my pocket a frog and two quarters and a baseball card and a pocket knife and a string." But when we get older, we learn that we don't need all the "and"s, so we use commas in place of the "and"s: "I have in my pocket a frog, two quarters, a baseball card, a pocket knife and a string." Since I kept the last "and" in the sentence, I didn't replace it with a comma. To me using ", and" is redundant, because they both do the same job. If I keep the "and" then I don't need the comma.
However, there are some cases where there's a clause, so I want to set off the clause with commas--in which case, a comma could come before the "and." Such as: "I have in my pocket a frog, two quarters, a baseball card, a pocket knife, the one my dad gave me for Christmas, and a string."