Commas are the most common way to set off such phrases, but they are not the only way. Most grammarians seem to think that em dashes or parenthesis are acceptable, and I've seen styles guides that recommend doing that if there are commas in the sentence.
As far as I can tell if we just stopped using commas to set off such phrases when other commas are in the sentence (or just stopped using commas to set off such phrases all the time) that would get rid of all the cases where including the Oxford comma in a list makes the list ambiguous, without changing the cases where not having an Oxford comma is ambiguous.
Is Betty a maid or did they go with 3 people?
Or better yet: "They went to Oregon with a cook, and maid named Betty."
†For example:
Betty, one maid, and one cook.
Betty, and a maid and a cook (a little awkward)
A maid, a cook, and Betty (depends on how you want Betty's inclusion to land for the reader)
I'm not saying the Oxford comma is bad. I'm just saying that it isn't 100% perfect as many people imply.