1150–1200; Middle English, orig. the second, unstressed member of correlative other … or, earlier other … other, Old English āther … oththe, ā-hwæther … oththe, for oththe … oththe either … or; cf. ay¹, whether

See and/or, either.

before 950; Middle English, Old English ār soon, early; cognate with Old Norse ār, Gothic air early; compare Old English ǣr soon, before, ere

1400–50; late Middle English < Middle French < Latin aurum gold

1940–45

< Latin; in some cases continuing Middle English -our < Anglo-French, Old French < Latin -ōr-, stem of -or, earlier -os

While the -or spelling of the suffix -or1 is characteristic of American English, there are occasional exceptions, as in advertising copy, where spellings such as colour and favour seek to suggest the allure and exclusiveness of a product. The spelling glamour is somewhat more common than glamor —not actually an instance of -or1 , but conformed to it orthographically in the course of the word's history. In British English -our is still the spelling in most widespread use, -or being commonly retained when certain suffixes are added, as in color ation, honor ary, honor ific, labor ious, odor iferous. The English of the Southern Hemisphere (Australia, New Zealand, South Africa) tends to mirror British practice, whereas Canadian English shares with the U.S. a preference for -or but with -our spellings as freely used variants.
The suffix -or2 is now spelled -or in all forms of English, with the exception of the word savior, often spelled saviour in the U.S. as well as in Britain, especially with reference to Jesus.

Middle English < Anglo-French, Old French -o(u)r < Latin -ōr-, stem of -or, extracted from -tōr -tor by construing the t as the ending of the past participle (hence Latin factor maker, equivalent to fac(ere) to make + -tor, was analyzed as fact(us), past participle of facere + -or); merged with Anglo-French, Old French -ëo(u)r < Latin -ātōr- -ator; cf. -eur