The adverb oftentimes is an unnecessary variant of often. While using it is not an error, exactly, the word always bears replacement with the shorter word. The same can be said of the less common ofttimes.
Oftentimes is most common in the U.S. and Canada. We can’t explain this, but it does seem to be a trend; the word occurs more frequently in U.S. sources than it did even a decade ago—though occurrence of the word is still nowhere near its 19th-century peak. For instance, in a large number of American news stories from the year 2010, often appeared about 200 times for every instance of oftentimes, whereas the ratio was closer to 500:1 in 2000. In British publications, in contrast, oftentimes appears about once for every 4,000 instances of often—or essentially not at all.
Of course, often is just a longer way of saying oft (which goes back to Old English, predating often by many centuries), but no one uses that form. English can be strange.
While oftentimes isn’t wrong, it could give way to the shorter often in cases like these:
Oftentimes, these business ventures net them more than their paychecks from the screen or stage. [Forbes]
A supermajority, typically of two-thirds of shareholders, is oftentimes required to approve a merger or acquisition. [National Post]
The crowd was quite animated, oftentimes cheering and oftentimes yelling out in disgust. [Captain Jack for President, John Jones]
But oftentimes, he has to acknowledge that his life is governed by a double standard. [AV Club]
Oftentimes they lack the resources to hire additional staff to help tackle necessary tasks. [Newsday]
Positive rap, like political rap, is oftentimes not as commercial as gangsta or materialistic rap. [Rap Therapy, Don Elligan]