When I write a list of something. What are the following correct in American style.
A, B, and C.
A, B and C.
Option 1--A, B, and C--is used more often in the US. It's called the serial or Oxford comma, and there are arguments for and against its use. However, US Government Printing Office Style Manual, APA Style Manual, MLA Style Manual, and Strunk and White's Elements of Style, all mandate the use of the serial comma. It is less frequently used in journalistic writing.
In a complex list with items within items, it's preferred to use the serial comma to avoid ambiguity. For example: John, Mary, Sue and Frank, and Jamie or John, Mary, Sue, and Frank and Jamie.
Wikipedia has a detailed overview of the use of serial commas.