Most scholarship ascribes the earliest known use of the name "Oregon" to a 1765 petition by Major Robert Rogers to the Kingdom of Great Britain, seeking money to finance an expedition in search of the Northwest Passage. The petition read "the rout... is from the Great Lakes towards the Head of the Mississippi, and from thence to the River called by the Indians Ouragon...."  Thus, the early Oregon Country and now the present day state of Oregon took their names from the river now known as the Columbia River. 
In 1766, Rogers commissioned Jonathan Carver to lead such an expedition and in 1778, Carver used Oregon to label the Great River of the West in his book Travels Through the Interior Parts of North America.  The poet William Cullen Bryant took the name from Carver's book and used it in his poem Thanatopsis, published in 1817, to refer to the recent discoveries of the Lewis and Clark Expedition, which helped establish the name in modern use. 
Why Rogers used the name has led to many theories, which include these:
- George R. Stewart argued in a 1944 American Speech article that the name came from an engraver's error in a French map published in the early 18th century on which the Ouisiconsink ( Wisconsin River) was spelled "Ouaricon-sint", broken into two lines with the -sint below, so that there appeared to be a river flowing to the west named "Ouaricon".   The theory was endorsed in Oregon Geographic Names as "the most plausible explanation". 
- In a 2004 article for the Oregon Historical Quarterly, Professor Thomas Love and Smithsonian linguist Ives Goddard argue that Rogers chose the word based on exposure to either of the Algonquian words wauregan and olighin, both meaning "good and beautiful (river)," but in Rogers' day referring to the Ohio River.  According to their theory, Rogers was inspired by a reference to a "Belle Rivière," or "Beautiful River" (which could have arguably stood for the river in question) in Antoine-Simon Le Page du Pratz's Histoire de la Louisiane, published in 1758.  Le Page du Pratz also referred to an alleged Indian origin for this name but did not supply an Indian word for it. Goddard and Love then surmise that Rogers substituted his bastardization of the Mohegan pidgin word wauregan for Le Page du Pratz's "translation" Belle Rivière. 
- John E. Rees in a 1920 article for the Oregon Historical Quarterly also ascribed the first use to Jonathan Carver, but hypothesized that the word derived from two Shoshone words, Ogwa (river) and Pe-On (west) which Carver had heard on his visits to Sioux Indians (where the Sioux pronounced gwa as an r). In other words, the word "Oregon" would mean something like "River of the West" in Shoshone. He does not trace the word back to Col. Rogers. 
- T.C. Elliott described the theory that the name was a corruption of the French word ouragan ( hurricane, windstorm, or tornado). 
- One theory suggests that "Oregon" is a corruption of the French Origan ( Oregano), a theory dismissed by Harvey W. Scott, a historian and early editor of The Oregonian.  According to Scott, it was "a mere conjecture absolutely without support. More than this, it is completely disproved by all that is known of the name."
- In 2001, archaeologist Scott Byram and David G. Lewis published an article in the Oregon Historical Quarterly arguing that the name Oregon came from the word oolighan (see eulachon), referring to grease made from fish, a highly prized food source for Native Americans of the region. Allegedly, the trade routes brought the term eastward.
- In 1863, Archbishop François Norbert Blanchet advanced the theory that the name derives from early Spanish settlers who referred to the big, ornamented ears of the region's native people by the name "Orejon." 
Others have speculated  that the name is related to the kingdom of Aragon: the major part of the Spanish soldiers who conquered the West Coast from California to Vancouver Island in the 18th century were in fact from Catalonia, a principality of the ancient Crown of Aragon in Spain. The name might also be derived from the Spanish last name Obregón, which in turn is related to a Spanish place name "Obregon" in Santander, Spain, on the north coast. 
- Archbishop Blanchet (March 8, 1884). "Origin of the name Oregon" (PDF). Portland Oregonian. reprinted by the New York Times.
- Oregon Almanac
- Article on Oregon State in the Catholic Encyclopedia of 1910. Retrieved 10 March 2012
- Elliot, T.C. "Jonathan Carver's source for the name Oregon," in: Oregon Historical Quarterly, Vol. XXIII (1922), pp. 58-69
- Rees, p. 326
- Did this 1715 map influence the first appearance of the name Oregon? Geographicus.com.
- George R. Stewart (1944). "The Source of the Name 'Oregon'". American Speech. Duke University Press. 19 (2): 115–117. doi: 10.2307/487012. JSTOR 487012.
- George R. Stewart (1967) . Names on the Land: A Historical Account of Place-Naming in the United States (Sentry edition (3rd) ed.). Houghton Mifflin. pp. 153, 463. ISBN 1-59017-273-6.
- Lewis A. McArthur; Lewis L. McArthur (2003) . Oregon Geographic Names (Seventh ed.). Portland, Oregon: Oregon Historical Society Press. ISBN 0-87595-277-1.
- Goddard and Love, par. 7
- Goddard and Love, par. 33
- Goddard and Love, pars. 34-41
- Rees, John E. (1 December 1920).
"Oregon—Its Meaning, Origin and Application". The Quarterly of the Oregon Historical Society. 21: 319. Retrieved 2013-04-14.
The word "Oregon" is derived from a Shoshoni Indian expression meaning, The River of the West, originating from the two Shoshoni words "Ogwa," River and "Pe-on," West, or "Ogwa Pe-on." The Sioux pronounced this word in the more euphonious manner in which their tongue excels and the Shoshoni "Gwa" underwent, etymologically, a variation in the new language and became changed to "r," thus giving the sonorous word which Jonathan Carver, who first published the name to the English world, heard spoken by them during his visit with the Sioux nation.[Boaz, Handbook of American Indian Languages, p. 875.]
- Elliott, T.C. (1921). "The Origin of the Name 'Oregon'". Oregon Historical Quarterly. by Oregon Historical Society: 91–115. Retrieved 2008-01-10.
- Scott, Harvey W. (1900).
Not Marjoram: The Spanish Word "Oregano" Not the Original of Oregon. Oregon Historical Quarterly. 1. Oregon Historical Society. p. 165.
- Galvani, W.H. "The early explorations and the origin of the name of the Oregon country," in: Oregon Historical Quarterly, Vol. XXI, No. 4 (Dec. 1920), pp. 332-340. Galvani tries to debunk the ascription to Carver and alleges an earlier use of the name on Spanish maps, citing one Gabriel Franchere.
- "Obregon Coat of Arms and History". House of Names. Retrieved 2012-07-08.
- Goddard, I and Love, Th.(2004) "Oregon, the Beautiful," in: Oregon Historical Quarterly, Vol. 195, No. 2 (2004)
- Rees, J.E. "Oregon - Meaning, Origin and Application", in Oregon Historical Quarterly, vol. XXI, No. 4 (Dec. 1920), pp. 317-331
- Horner, John B. (1921). Oregon: Her History, Her Great Men, Her Literature. Portland: The J.K. Gill Co.