 I've  noticed that people under about the age of 50 tend to pronounce "often" with a hard "t". Older people tend to leave the "t" silent, pronouncing it "offen".  I attribute this to the prevalance of grade school phonics lessons, but really have no idea.  Pronouncing the "t" adds complexity to the word and sounds slightly pretentious to my ear.  What do think?
It does sound pretentious with the t, though it seems I hear t just as often as not, from young and old.
But never hear 'oftentimes' different from the one way- it would just produce too many spits from the 2 t's so close together.
By the way I never hear t in 'listen' either; wonder how far back since the t in there went silent?
There are many words with silent t in similar phonetic context: fasten, soften, hasten, chasten, listen, etc.
Chestnut?
In my viewpoint pronouncing 't' in "often" sounds informal .