As a native English speaker, the word "oftentimes" sounds very unusual to me. I sometimes hear it being used in radio interviews etc. by people who use english as a second language (quite commonly Germans). I was wondering why this might be - is it commonly taught in textbooks, or perhaps a direct translation of a word commonly used in other languages?
It may sound "unusual" to you because it is both archaic and predominantly used in North America.
So, if you are British, the word oftentimes would then sound twice as odd to you as it would to someone from the U.S. or Canada.
It's a native word, used not only by learners but by native speakers. (Shakespeare uses it, for example!) I first began noticing an increase in its use here in Toronto, Canada around 2009 or 2010; you hear an odd word like that once or twice and then suddenly every radio interviewee is using it.
Buzzwords like that spread like wildfire and are picked up by any attentive listener, including language learners. (Another example from a couple of years later, at least here in Toronto, is "cognizant" used as a synonym of "aware".)
I suspect that when words are replaced by these longer synonyms (synonymous in everyday usage, anyway), it's a subconscious attempt to inflate diction or follow language trends for social reasons.
There's no reason not to use it nowadays, at least (if the Oxford Dictionaries link @User26328 shared is correct) in North America. But there's also no reason not to use "often".