What is an Oxford comma and why do editors have such strong feelings about their use?
An Oxford comma is a comma before the last item in a series or list. For example take the following two statements:

I was chased by the fox, the bear and the dog.
I was chased by the fox, the bear, and the dog.

The meaning is the same but the second statement contains an Oxford comma.

Some say the Oxford comma adds clarity to some statements. This is apparent when there can be ambiguity between a list and an appositive. Take the following two example statements:

We went to dinner with our coaches, Chris and Terry.
We went to dinner with our coaches, Chris, and Terry.

The first statement can appear to imply that we went to dinner with two people whose names are Chris and Terry who are both our coaches. There is no such implication that can be extracted from the second statement. The second statement implies we went to dinner with at least 4 people: Chris, Terry, and at least two coaches. This type of ambiguity is what proponents of the Oxford comma latch onto in its defense.

On the other hand, opponents of the Oxford comma claim it should be avoided in favor of rewording sentences when necessary. The sentence:

We went to dinner with our coaches, Chris, and Terry.

Can easily be reworded as:

We went to dinner with Chris and Terry, as well as our coaches.

The "as well as" language is claimed to be more clear than the Oxford comma albeit less concise. Editors who oppose the Oxford comma typically privilege clarity over conciseness.