A lot vs. Alot: 9 Grammatical Pitfalls
1530s, interjection expressing various emotions, a common Indo-European word (e.g. Old French ô;, oh; Latin o, oh; Greek o; Old Church Slavonic and Lithuanian o; Gothic, Dutch, German o; Old Irish a; Sanskrit a), but not found in Old English, which translated Latin oh with la or eala.
The present tendency is to restrict oh to places where it has a certain independence, & prefer o where it is proclitic or leans forward upon what follows .... [Fowler]Often extended for emphasis, e.g. Oh, baby, stock saying from c.1918; oh, boy (1910); oh, yeah (1924). Reduplicated form oh-oh as an expression of alarm or dismay is attested from 1944. Oh-so "so very" (often sarcastic or ironic) is from 1922. Oh yeah? "really? Is that so?" attested from 1930.