I have posted on these wonderful commas in the past. You can find the explanations of why to use them in other blog postings, but this is the Readers’ Digest condensed version for those of you in a hurry.
Oxford commas are used to connect the second-to-the-last item in a list of things. For example, here is a sentence without an Oxford comma:
“I love my parents, Daisy Duck and Daffy Duck.” What? Your parents are ducks???? Let’s use an Oxford comma to clear up the sentence:
“I love my parents, Daisy Duck, and Daffy Duck.” Okay, now you love three different individuals. That’s what you were going for. Well done!
Parenthetical expression commas are used when you, for whatever reason, want to dump something extra into the middle of your sentence. (Like I just did.) If you want use one parenthetical comma, you must use two. One goes in front of the parenthetical expression, to be clear, and one goes after the expression. (Like I just did, again.) You can’t use just one, like you cannot eat just one Lays potato chip.
Introductory phrases should have commas. These are the little comments you make at the beginning of a sentence. If you use too many, your writing will become dull. (There was one in the previous sentence.) I have another blog posting that goes into detail on how to get rid of them. Some of my students will use introductory phrases in 4 out of 5 sentences. That is dreadful.
I hope this helps!