-or1

1.
a suffix occurring in loanwords from Latin, directly or through Anglo-French, usually denoting a condition or property of things or persons, sometimes corresponding to qualitative adjectives ending in -id4, (ardor; honor; horror; liquor; pallor; squalor; torpor; tremor); a few other words that originally ended in different suffixes have been assimilated to this group (behavior; demeanor; glamour).
Origin
< Latin; in some cases continuing Middle English -our < Anglo-French, Old French < Latin -ōr-, stem of -or, earlier -os
Usage note
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 coloration, honorary, honorific, laborious, odoriferous. 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.

-or2

1.
a suffix forming animate or inanimate agent nouns, occurring originally in loanwords from Anglo-French (debtor; lessor; tailor; traitor); it now functions in English as an orthographic variant of -er1 , usually joined to bases of Latin origin, in imitation of borrowed Latin words containing the suffix -tor, (and its alternant -sor). The association with Latinate vocabulary may impart a learned look to the resultant formations, which often denote machines or other less tangible entities which behave in an agentlike way: descriptor; plexor; projector; repressor; sensor; tractor .
Origin
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
British Dictionary definitions for -or

-or1

suffix
1.
a person or thing that does what is expressed by the verb: actor, conductor, generator, sailor
Word Origin
via Old French -eur, -eor, from Latin -or or -ātor

-or2

suffix
1.
indicating state, condition, or activity: terror, error
2.
the US spelling of -our
Word Origin and History for -or

word-forming element making nouns of quality, state, or condition, from Middle English -our, from Old French -our (Modern French -eur), from Latin -orem (nominative -or), a suffix added to pp. verbal stems. Also in some cases from Latin -atorem (nominative -ator).

In U.S., via Noah Webster, -or is nearly universal (but not in glamour, curious, generous), while in Britain -our is used in most cases (but with many exceptions: author, error, senator, ancestor, horror etc.). The -our form predominated after c.1300, but Mencken reports that the first three folios of Shakespeare's plays used both spellings indiscriminately and with equal frequency; only in the Fourth Folio of 1685 does -our become consistent.

A partial revival of -or on the Latin model took place from 16c. (governour began to lose its -u- 16c. and it was gone by 19c.), and also among phonetic spellers in both England and America (John Wesley wrote that -or was "a fashionable impropriety" in England in 1791).

Webster criticized the habit of deleting -u- in -our words in his first speller ("A Grammatical Institute of the English Language," commonly called the Blue-Black Speller) in 1783. His own deletion of the -u- began with the revision of 1804, and was enshrined in the influential "Comprehensive Dictionary of the English Language" (1806), which also established in the U.S. -ic for British -ick and -er for -re, along with many other attempts at reformed spelling which never caught on (e.g. masheen for machine). His attempt to justify them on the grounds of etymology and the custom of great writers does not hold up.

Fowler notes the British drop the -u- when forming adjectives ending in -orous (humorous) and derivatives in -ation and -ize, in which cases the Latin origin is respected (e.g. vaporize). When the Americans began to consistently spell it one way, however, the British reflexively hardened their insistence on the other. "The American abolition of -our in such words as honour and favour has probably retarded rather than quickened English progress in the same direction." [Fowler]