Hyphens, En Dashes, Em Dashes

Q. How would you treat “over apologize” in this sentence? “Be careful not to over apologize.”

A. Over is a prefix in that case, so close it up: “Be careful not to overapologize.” Please see CMOS 7.89, section 4 (“Words Formed with Prefixes”), under the prefix over.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]