- O'Hara & Zeese Civil Engineers and Surveyors (Aitkin, Minnesota; The Dept. (Minnesota) (1)
- O.W. Gray & Son (Philadelphia), 1897 (1)
- O.W. Gray? (Philadelphia) (1)
- O?sterreichische Waffenfabriks-Gesellschaft (Wein) (3)
- OSS Repro. Section (Washington, D.C.?) (1)
- Office of City Engineer (Columbus, Ga) (1)
- Office of City Engineer (New Castle, Pa) (1)
- Office of Indian Affairs (Washington, D.C) (1)
- Office of Press Print. Co. (Saint Paul, Minn) (1)
- Office of Surveyor General of India ; Calcutta (1)
- Office of Village Engineer (Hibbing, Minn) (2)
- Office of the Chief of Engineers (Washington, D.C) (1)
- Office of the Chief of Engineers U.S.A (Washington) (1)
- Office of the Chief of Engineers U.S.A (Washington?) (2)
- Office of the Chief of Engineers, U.S. Army (Washington, D.C.?) (1)
- Office of the Chief of Engineers, War Dept (Washington, D.C.?) (2)
- Office of the City Engineer? (Newark, Ohio?) (1)
- Office of the Engineer (Jackson, Mich) (1)
- Office of the Supt. Grand Trunk Road (Allahabad) (2)
- Office of the Supt. Grand Trunk Road; Allahabad (2)
