You can pronounce it either way. The pronunciation changes depending on the person, but I think it is probably more common that you will hear [ɔ́ː*f*ən].

While both pronunciations are acceptable, keep in mind that there might be individuals who will "correct" you when you don't pronounce the "t" (in the most unreasonable of cases, you might even be seen as "uneducated"). Also, you do pronounce the "t" in the archaic version of "often", which is "oft", and I believe the same is done for words like "oftentimes". I'd say pick your flavour and be aware of the people around you as such a minuscule thing could potentially lead to annoying situations/conversations.

you should pronounce it as 'ofun'