[Andreas Brekken]

Tr.O is as Chris stated an abbreviation for Truppenoffizier, an officer that had gone the usual track through the Kriegsschule courses to become a professional officer.

The other possible variant is Kr.O or Kriegsoffizier, personnel that were promoted to officers rank because of their actions during wartime. (some of these also attended officer courses of different kinds)

Regards,
Andreas