An Oxford comma is a comma that precedes the conjunction after the penultimate item in a series (ex: She ate rice, beans, and tomatoes for dinner). There is debate whether or not the Oxford comma is necessary, and while some people consider omission of the Oxford comma acceptable, always include the Oxford comma on the ACT.
Dashes are used primarily to enclose information within a sentence. They are versatile and can be used in place of commas, parenthesis, and colons. However, the primary use of a dash is to emphasize or highlight information, so it is important to use dashes sparingly. They provide the emphasis that commas, parentheses, and colons do not provide.
Semicolons are usually used between related independent clauses. It can also be used to divide items in a series where the items themselves contain commas.