- Ontario Ministère de l'agriculture, de l'alimentation et des affaires rurales,
- Ontario. Royal Commission on Forest Reservation and National Park
- Ontario. Special Committee on Game Situation.
- Ontario. Task Force on Off-Track Betting
- Ontario. Waste Management Branch.
- Ontario. Water Quality Branch.
- Ontario. Water Resources Branch.
- Ontario. Water Resources Branch. Environmental Services.
- Ontiveros, D
- Ontiveros, Rebeca
- Onto, J G
- Onuferko, Thomas
- Onuferko, Tom
- Onuki, Maria K.
- Onus, M L
- Onyango, D W
- Onyschuk, I
- Oo, San Lwin
- Oo, Thaung
- Ooi, E L
- Ooi, Mark
- Ooi, Mark K J
- Ooi, Xin Wei
- Ooi, Yue-Shin
- Ooishi, Shigeko
- Ooka, Hiroshi
- Ookubo, Y
- Ool, Ries van
- Oolooyuk, David
- Oomizu, Souichi
- Oommen, Mathew M
- Oommen, Meera Anna
- Oommen, Oommen V
- Oommen, V P
- Oonuki, Makoto
- Oort, Abraham H.
- Oort, E. D. van (Eduard Daniël), 1876-1933
- Oortmerssen, G A
- Oosenbrug, S.
- Oosenbrug, S.M.
- Oostendorp, Cora
- Ooster, William Alexandre, 1816-
- Oosterbeek, Kees
- Oosterbroeck, P
- Oosterbroek, P
- Oosterbroek, Piotr
- Oosterbroek, Pjotr
- Oosterdyk, Nicolai Georgii.
- Oosterhof, Jessica
- Oosterhout, Cock van, 1968-
- Oosterhuis, H. T.
- Oosterom, María V. López van
- Oosterwyk, Jan.
- Oosterwyk, Johannes, active 1700-1737
- Oosthuizen, J. H., 1994
- Oosting, D P
- Oosting, Henry J
- Oosting, Henry John, 1903-
- Ooststroom, S. J. van
- Oota, Tadachika
- Oota, Yoshihiko
- Ootsubo, Takahiro
- Oparin, Aleksandr Ivanovich, 1894-1980
- Opatova, Vera
- Opatovsky, Itai
- Opatowski, D Guilelmo
- Opazo, Juan C.
- Opazo, L. Felipe
- Opazo, Sergio
- Opdyke, Neil D.
- Opel, Matthew R
- Opell, Brent D.
- Open Book Publishers,
- Open Court Publishing Company.
- Operschall, P
- Opesso, Gianna.
- Opgenoorth, Lars
- Opheim, Boyd R.
- Opheim, M
- Ophel, Ivan L.
- Ophige, Michael
- Ophof, A J
- Ophthalmological Society of the United Kingdom. Library,
- Opic, Pierre
- Opie, A M
- Opisa, Selpha
- Opit, G P
- Opitz, Scott T.
- Opitz, Weston
- Opiz, Philipp Maximilian.
- Opler, Paul A.
- Oplinger, Anne A.
- Oplustil, Stanislav
- Opoix, M. (Christophe) 1745-1840
- Opoix, Octave.
- Opoku, Albert Mawere.
- Oporto, Jorge
- Oporto, L. T.
- Oporto, Samuel
- Opp, Susan B
- Oppe, Joseph
- Oppel, Albert Carl
- Oppel, Albert, 1863-1915
- Oppel, Albert.
- Oppel, Carl Albert, 1831-1865
- Oppel, M
- Oppel, Marissa
- Oppel, Marissa N
- Oppel, Marissa N.
- Oppel, Michael, 1782-1820
- Oppel, Steffen
- Oppen Greenhouses,
- Oppenheim, P
- Oppenheim, Paul, 1863-1934
- Oppenheim, R S
- Oppenheim, Samuel
- Oppenheimer, Carl H.
- Oppenheimer, Carl, 1874-1941
- Oppenheimer, David, 1834-1897
- Oppenheimer, Hank
- Oppenheimer, Heinz
- Oppenheimer, Hillel, 1899-1971
- Oppenheimer, J R
- Oppenheimer, J. J.
- Oppenheimer, Jane M
- Oppenheimer, Marina
- Oppen'S Greenhouse,
- Opper, B
- Opper, Richard Henry, 1952-
- Opperman, C. L.
- Oppert, Brenda
- Oppian, fl 2nd cent-
- Oppolzer, Th. v. (Theodor), 1841-1886
- Oppong, James
- Oppong-Mensah, Birgitta
- Oprasertsawat, Panut
- Oprea, Adrian
- Oprea, M.
- Oprea, Monik
- Oprecht, E
- Opresko, D M
- Opresko, D M
- Opresko, Dennis
- Opresko, Dennis M
- Opršal, Zdeněk
- Opstaele, Marc van
- Opytnoe pole (Odesa, Ukraine)
- Oquendo, Oviana
- O'quinn, Robin
- Oquossoc Angling Association.
- Oradovskaya, M M
- Oraie, Hamzeh
- Oral, Münevver
- Oram, D A
- Oram, Keith
- Oram, Ryan
- Orams, M B
- Oran, S
- Oran, Seyhan
- Oranday Cárdenas, A
- Oranday, MA
- Oranday-Cárdenas, A.
- Orange County Fern Growers Association.
- Orange County Fern Growers' Association.
- Orange County Nurseries.
- Orange County Nursery and Land Company.
- Orange County Nursery Co. (Anaheim, Calif.)
- Orange County Nursery Co.,
- Orange County Nursery,
- Orange County Ornamental Nursery.
- Orange Hill Nursery and Fruit Orchards.
- Orange Judd & Company.
- Orange, Paul
- Orantes, Carolina
- Oraze, Michael J
- Orazio, Christophe
- Orbach, Eylon
- Orbigny, Alcide Dessalines d', 1802-1857
- Orbigny, Charles d', 1806-1876
- Orbigny, H D'
- Orbigny, Henri d' 1845-1915
- Orcan, Nermin
- Orcel, Jean
- Orces V., Gustavo,
- Orces-V, G
- Orchadotte Nurseries.
- Orchadson, I. Q.
- Orchard & Garden Supply Company.
- Orchard Hill Nursery.
- Orchard Home Nursery.
- Orchard Nursery,
- Orchard, A E
- Orchard, Ae
- Orchard, Anthony E
- Orchard, F. P.
- Orchard, M. J.
- Orchard, Stan
- Orchard, T A
- Orchard, William C.
- Orchardson, I. Q.
- Orchid Conference South Kensington) 1885-
- Orchid Exchange (Philadelphia, Pa.)
- Orchid Gardens.
- Orchid House
- Orchid Ranch, Inc.,
- Orchid Research Co.,
- Orchid Society of New South Wales.
- Orchidéenne, société d'amateurs d'orchidées.
- Orchids By Turner, Inc.,
- Orchids By Wilson.
- Orchidvale,
- Orchidwood (New Rochelle, N.Y.)
- Orchidwood Gardens.
- Orchidwood, Inc.
- Orchiston, W
- Orchiston, Wayne
- Orchymont, A D
- Orchymont, A D'
- Orci, Kirill
- Orci, Kirill Márk
- Orcival, J
- Orcutt Co. (Orcutt, Calif.)
- Orcutt Seed and Plant Company.
- Orcutt, Alfred Walter.
- Orcutt, Barbara S
- Orcutt, Charles Russell, 1864-1929
- Orcutt, Eliza E.
- Orcutt, I. H.
- Orcutt, Mary Logan
- Ord, George
- Ord, George, 1781-1866
- Ord, Richard E
- Ord, Richard, 1849-1920
- Ord, Richard.
- Ord, W Michael
- Ord, William M. (William Miller), 1834-1902
- Ordano, M
- Ordano, Mariano
- Ordaz Silva, Salvador
- Orde, C J
- Orde-Browne, Granville St. John, Sir, 1883-1947
- Ordóñez Ch, María Fernanda
- Ordóñez de Ceballos, Pedro, ca 1545-ca 1630
- Ordoñez Mazier, Diego
- Ordoñez Trejo, Eduardo Javier
- Ordóñez, Ezequiel, 1867-
- Ordoñez, Reina
- Ordóñez-Delgado, Leonardo
- Ordóñez-García, Magali
- Ordóñez-Garza, Nicté