The Athletics, of course, are more commonly referred to as the Oakland A’s. No doubt, there are style guides that advise that plurals of single letters always require ’s, but neither common sense nor the style guide I usually consult, The Chicago Manual of Style (CMOS), advocates such usage.
Apostrophes most commonly indicate either a possessive case (as in George’s) or the elision of letters (as, for example, in contractions like don’t). An apostrophe is also used to indicate a simple plural when the letter involved is lowercased (as in a’s), since, in the absence of the apostrophe, the combination of letters could be mistaken for another word (as, in the example).
Here is the main part of the entry on the topic from the 16th edition of CMOS (7.14, p. 353):
Plurals for letters, abbreviations, and numerals. Capital letters used as words, numerals used as nouns, and abbreviations usually form the plural by adding s. To aid comprehension, lowercase letters form the plural with an apostrophe and an s.When referring to the team as the Oakland A’s, A’s acts as a noun, and there is no possession implied. Oakland A’s is the semantic equivalent of Oakland Athletics. This suggests a rather farfetched justification for the use of the apostrophe, namely, that A’s could be an abbreviation for Athletics, with the apostrophe standing in for the omitted thletic. This is not reasonable because A is itself an abbreviation (or nickname) for Athletic. Just as the Pittsburgh Pirates comprise players, each of whom is a Pirate, the Oakland Athletics comprise players, each of whom is an Athletic, or, if you prefer, an A. (One can find examples of players described as an Athletic or an A.) This usage for the Oakland team seems odd only because Athletic is not usually a noun, so the locution is usually avoided. The team traces its history to the Philadelphia Athletics and the athletic clubs of the late 19th century, whose members were apparently called athletics.
All of this is to say that the Oakland A’s should really be the Oakland As, but I don’t expect this little essay to change anything. I suspect that, when A’s was first used, use of the apostrophe in such situation was common. More modern usage has become simplified and more logical.
Interestingly, plural team names never seem to be used in the possessive case. One never sees anyone writing about the Pirates’ manager, only about the Pirates manager. Likewise, no one reads about the Yankees’ owner or, God forbid, the A’s’ owner!
Errors involving apostrophes are common. I recently received a mailing from Family Video that included the slogan “Home of the Free Kids Movies!” This seems to refer to movies about kids, but what is meant is movies for kids. The slogan should have been “Home of the Free Kids’ Movies.” Of course, we speak of “adult movies.” Should we not, by analogy, speak of “kid movies”?