1883, alt. of
hallo (1840), itself an alt. of
holla, hollo, a shout to attract attention, first recorded 1588. Perhaps from
holla! "stop, cease." Popularity as a greeting coincides with use of the telephone, where it won out over Alexander Graham Bell's suggestion,
ahoy. Central telephone exchange operators were known as
hello-girls (1889).
"Hello, formerly an Americanism, is now nearly as common as hullo in Britain (Say who you are; do not just say 'hello' is the warning given in our telephone directories) and the Englishman cannot be expected to give up the right to say hello if he likes it better than his native hullo. [H.W. Fowler, "A Dictionary of Modern English Usage," 1926]
[he-loh, huh-, hel-oh] 
interjection, noun, plural -los, verb, -loed, -lo⋅ing.
oʊ