often vs oftentimes

often and oftentimes both are adverbs.

Word	Noun	Adjective	Verb	Adverb
often	No	No	No	Yes
oftentimes	No	No	No	Yes

As adverbs, often and oftentimes are synonyms defined as:

Other synonyms of often include frequently, oft, ofttimes.

often (adverb)	oftentimes (adverb)
many times at short intervals	many times at short intervals
frequently or in great quantities
in many cases or instances