What is the difference between often and oftentimes, and is oft a word? The short answers are that there is no difference, and yes.
These three adverbs all stem from the Old English (and Middle English) term oft, meaning “frequently.” The longer variants developed in the 1300s. Just as often is an extended alteration of oft (likely invented to ease the transition to a word beginning with a vowel), oftentimes derived from ofttimes.
That last word is all but unknown in Modern English, and even oft is rare but survives in a saying from William Shakespeare’s Julius Caesar, “The evil that men do lives after them. The good is oft interred with their bones.” It is also used in combination with verbs in constructions such as oft-praised and oft-told.
Often and oftentimes are interchangeable, but the more archaic-sounding latter word is less economical and has an obsolete taint equivalent to that of the superfluous -st ending in words such as amidst and amongst or the extraneous first syllable of upon. (Interestingly, against differs from its cousins amidst and amongst in that the truncated form again is not a variation but a word with a distinct meaning.)
The antonym seldom, meaning “rarely,” which also comes from Old English, originally had a compound -times form as well (though it was hyphenated), and seldhweanne (“seldwhen”) and seldsiene (“seldseen”) were part of the word-hoard, though only the latter word evolved into a later form (seldom-seen). (The variant seld-shown appeared in Shakespeare.)