Haven't been up with the online Opeth community for a long while, but was on the origonal Opeth mailing list for a number of years ending in 2000 back when Mikael occasionally posted, and there was a thread on this.
The verdict there was "Oh-pith", with the first syllable long and the second almost swallowed, for what it's worth.
Yeah, my dad can't pronounce TH sounds, so he calls them Opec. I've noticed a lot of people call then opith or oputh instead of oh-peth like i say it...
i say oh-peth, as in death
but if i get to meet mikael, i'll ask him, and say something like
on a lot message boards people always ask how to pronounce your, and the band's name, would you mind explaining, thus putting an end to the question?