Type-O in FGO Songs

It took me a while to catch the Type-O thing... haha.

Yeah, The "For Guru's Only" is wrong, but "AAA's" is not necessarily wrong. You wouldn't type, "I received three As on my exams." As could be mistaken for as. Things like that commonly use an apostrophe, especially if there is a chance for misreading it. "I received three A's on my exams" is much more clear. I guess in the case of "AAAs" vs "AAA's," it could go either way because we all (should) know what an AAA is, and it wouldn't really cause any confusion.

Anyways, doesn't the same typo appear in "For Master's Only?" And since when is it "type-O" instead of "typo?"

I see people do this all the time. "How many AAA's do you have?" GRRRRR. Hahaha.

Use in forming certain plurals
An apostrophe is used by some writers to form a plural for abbreviations, acronyms, and symbols where adding just s rather than 's may leave things ambiguous or inelegant. While British English formerly endorsed the use of such apostrophes after numbers and dates,[citation needed] this usage has now largely been superseded. Some specific cases:

It is generally acceptable to use apostrophes to show plurals of single lower-case letters, such as be sure to dot your i's and cross your t's. Some style guides would prefer to use a change of font: dot your is and cross your ts. Upper case letters need no apostrophe when there is no risk of misreading: I got three As in my exams,[23] except possibly at the start of a sentence: A's are the highest marks achievable in these exams.

For groups of years, the apostrophe at the end cannot be regarded as necessary, since there is no possibility of misreading. For this reason, most authorities prefer 1960s to 1960's[23] (although the latter is noted by at least one source as acceptable in American usage[24]), and 90s or '90s to 90's or '90's.

The apostrophe is sometimes used in forming the plural of numbers (for example, 1000's of years); however, as with groups of years, it is unnecessary: there is no possibility of misreading. Most sources are against this usage.

The apostrophe is often used in plurals of symbols. Again, since there can be no misreading, this is also regarded[who?] as wrong.[23] That page has too many &s and #s on it.

Finally, a few sources accept its use in an alternative spelling of the plurals of a very few short words, such as do, ex, yes, no, which become do's, ex's, etc.[25] In each case, dos, exes, yeses (or yesses) and noes would be preferred by most authorities. Nevertheless, many writers are still inclined to use such an apostrophe when the word is thought to look awkward or unusual without one.