Medical officer was a UNIT rank. (AUDIO: The Helliax Rift)
Harry Sullivan was seconded to UNIT (PROSE: Harry Sullivan's War) to serve as Medical Officer shortly before the Third Doctor's regeneration. (TV: Planet of the Spiders, Robot)
Gemma Corwyn was medical officer of the Wheel. (PROSE: The Wheel in Space)
Marne was a medical officer on Thoros-Beta. (PROSE: Mindwarp)