Headlines and Titles of Works

Q. What is the correct way to write “four o’clock” as a book title?

A. Treat the contracting apostrophe in “o’clock” as if it were a hyphen and write Four O’Clock—capital O and C. The O is capitalized as the first word in a compound term, and the C because clock is a noun. If it helps, think of the title as short for “Four Of-the-Clock” (rather than “Four of the Clock”—​though the latter is how the term would normally be spelled out).

See also CMOS 8.161, which explains Chicago’s rules for capitalizing hyphenated compounds in titles.