Should "office mate" be one word or two?

Solution 1:

It's a noun-noun compound, and they vary in spelling a lot. Officemate, office-mate, office mate, roommate, bed mate, mincemeat, mince-meat, minced meat, etc.

There is no official spelling for such compounds, which are formed ad libitum in spoken English. Whatever looks good to you will work; English readers are mostly cooperative.

If anyone objects, you can write it off as being simply peevage.

Solution 2:

Merriam-Webster lists roommate as a word, but neither officemate nor office mate is listed. (My spell-checker flags officemate as wrong.)

As the following Ngram shows, office mate outstrips officemate in usage. (Many of the occurrences of officemate are accounted for by software products of that name.)

Solution 3:

The OED has an entry called "office-man" that does not appear to be hyphenated in its later uses.

ˈoffice-man †1.1 An officer; an official. Sc. Obs. 1908 J. M. Sullivan Criminal Slang 17 Office man, headquarters detective.    1921 Daily Colonist (Victoria, B.C.) 12 Oct. 16/2 (Advt.), Wanted—position as office man, watchman, warehouse, or place of trust.    1949 Partridge Dict. Underworld 479/1 Office man, a headquarters detective.