Who's This? Pulled from the pages of history, or if you like, page 18 of Who's Who vol. XVII.
How you could have heard of it: The Office of Strategic Services is the real-life precursor to the C.I.A., so you might have come across it a number of ways. But in DC Comics specifically, it became the branch of Task Force X known as Argent in later years, and was mentioned here and there in the post-Crisis universe, most notably in Secret Origins #14 and Suicide Squad Annual #1.
Example story: G.I. Combat #233 (1981) by Robert Kanigher and Eufronio Reyes Cruz
As you can see from the splash page, Fleur is really Mata Hari's secret daughter. The famous German spy/femme fatale is shot before our eyes in what looks like a historical comics story. Mata Hari's last words are however different from those recorded by history:
O.S.S. was basically a framework in which Kanigher (and a few other writers) could tell World War II spy stories. At its core, it was really an anthology series. Simple, but brutal, with art by the likes of Estrada and Cruz. I wouldn't mind reading (or re-reading; I got a lot of 80s G.I. Combat for the Haunted Tank stuff) more of those stories. I've got to find the one about Control's wife (Dina). It sounds really harsh!
Who else? The best-known character in vol.17 of Who's Who is also one of its most obscure. Who am I talking about?