[nelsonweiss]

The problem being alluded to is that adding an Oxford comma makes it appear as if "a policeman" is a parenthetical phrase describing George.

"Through the window she saw George, a policeman, and several onlookers."

Does she see two men, the policeman and George, as well as several onlookers? Or does she see one man, George the policeman, and several onlookers?

[lloyddobbler]

Agreed. As shown in my favorite illustration of an Oxford Comma dilemma ever: http://i.imgur.com/5LdZT.jpg

[salvadors]

Except that if you really meant the second case, you'd use a colon, not a comma.