OFF-uhn? Or OFF-tuhn?
Here’s Paul Brians:
People striving for sophistication often pronounce the T in this word, but true sophisticates know that the masses are correct in saying “offen.”
But here’s what the American Heritage Dictionary of the English Language (fourth edition) has to say about it:
In the 15th century, English experienced a widespread loss of certain consonant sounds within consonant clusters, as the (d) in handsome and handkerchief, the (p) in consumption and raspberry, and the (t) in chestnut and often. In this way the consonant clusters were simplified and made easier to articulate. With the rise of public education and literacy and, consequently, people’s awareness of spelling in the 19th century, sounds that had become silent sometimes were restored, as is the case with the t in often, which is now frequently pronounced. In other similar words, such as soften and listen, the t generally remains silent.
So that’s one against the pronounced t; one for. I reckon we need a tiebreaker. Let’s see what Mr. Fowler has to say about it:
The sounding of the t, which as the OED says is ‘not recognized by the dictionaries’, is practised by two oddly consorted classes—the academic speakers who affect a more precise pronunciation than their neighbours’…& the uneasy half-literates who like to prove that they can spell….
Case closed. Offen it is, then.