Should "office mate" be one word, in the same vein as "roommate"? I haven't been able to find any reference that addresses this.
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.
Merriam-Webster lists roommate as a word, but neither officemate nor office mate is listed. (My spell-checker flags officemate as wrong.)
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.
I have a feeling that there is a tendency to gradually drop hyphens and move towards a hyphenfree (hyphen-free/ hyphen free) English.
I would make a decision applying a similar perspective as with the word roommate. By separating the words, office and mate, it seems to emphasize the mate part, which is not the intent in an office context, just like it isn't in a co-habitation context e.g. room mate.
Secondary consideration: Corporate usage. There are numerous examples of major businesses named officemate, a single word. See OfficeMate:
the world's major provider of practice management software for eyecare practices
as well as Officemate International Corporation (OIC):
the leading manufacturer of office essentials...sold at thousands of office product suppliers all over the world.
*This is rather subjective, but I noticed that a prior answer already provided the full ngram treatment.
Listen to how the word is pronounced. If you hear separate stress on "OFF-ice" and "MATE", then you should certainly treat it as two words.
If you hear only one stress ("OFF-ice-mate"), and if you hear it very often, then you can try writing it as "office-mate". If nobody corrects you then, congratulations, you have played a part in the formation of a new English word.