When referring to letters of the alphabet as letters (e.g., the i in italics), an apostrophe is often used to form the plural:

“Impeccable is spelled with two c‘s.”

For capital letters, an apostrophe is not always needed:

“Jay has been learning his ABCs.”

Thus, the following sentence can be written with or without apostrophes:

For my final grades, I received three As and two Bs.

For my final grades, I received three A’s and two B’s.

Neither style is “incorrect,” but be consistent! (Don’t write: For my final grades, I received three A’s and two Bs.)

Check with the style guide your employer or publisher uses, or, if you are writing about people or organizations—or teams—follow their preference:

The Oakland Coliseum is home to the Oakland A’s. (not the Oakland As.)

As with letters, an apostrophe may or may not be needed when forming the plural of numerals; the choice, again, depends on which style guide is followed. (The style used for newspapers and magazines may differ from book publishing, for example.)

I counted six 1s and five 2s.

I counted six 1’s and five 2’s.

Just as numbers can be written as numerals (1, 2, 3) or spelled out as words (one, two, three), the names of letters are sometimes spelled out:

The ex should be printed in red. (The x should be printed in red.)

The Brits prefer zed to zee.

(Oh, however, would refer to zero: “We beat them, six–oh.”)

The plural of o is o’s or os.

In general, apostrophes are not used when forming the plural of words or acronyms:

I counted six ones and five twos.

The store sells used CDs and DVDs.

The boos drowned out the cheers.

My exes are all remarried now.

I was surrounded by oohs and aahs as I set up my tripod to photograph the fireworks.

Entrepreneurs (and record companies) don’t always get the punctuation right when choosing the names of their businesses (and albums). Compare:

with:

The second, Oohs and Aahs, is correct—no apostrophe required.

But what if Ooh and Aah are the names of the owners?

Then the correct punctuation would be:

“I’ll meet you at Ooh and Aah’s.”

(See my previous post about compound possession.)