After some digging I learned O.S.S. was an early offering of Lew Grade's ITC. From WikiZero:
O.S.S. was a Buckeye Productions and Associated Television co-produced wartime television drama series.
It ran for 26 half-hour monochrome episodes during the 1957–1958 season and was distributed by ITC Entertainment and networked in the United States by ABC.
The series followed the adventures of Frank Hawthorne, an agent with the American Office of Strategic Services, who operated behind Nazi lines in occupied France.
Main cast:
Ron Randell as Captain Frank Hawthorn
Lionel Murton as The Chief
Robert Gallico as O'Brien
Guest stars included Lois Maxwell, Christopher Lee and Doctor Who villain Roger Delgado.
Link to the book:
T. V. Picture Stories - O.S.S. Operation Sweet Talk