(a) When our perek speaks about olom haba it seems to be speaking about the olom haba after the moshiach comes and after techiyat hameysim. It doesn't seem to refer to the olom haneshamot where peoples' neshamot go after death. Menashe Hamelech appeared in a dream and spoke about the taava of avoda zara in his time. His neshama appears to be around.
(b) Is anyone excluded from the olom haba of the olom haneshamot? Who would be excluded and what happens to their neshamot? Are any neshamot destroyed completely? Does anyone stay in gehinnom more than 12 months or indefinitely?
(a) As we explained at length in our Insights, many commentaries explain that our Mishnah is indeed referring to the Olam ha'Ba after Techiyas ha'Mesim (as can be inferred from the Gemara's explanation for why one who denies Techiyas ha'Mesim has not Chelek in "Olam ha'Ba"); see also Ramban Vayikra 18:29. However, the *Rambam* (Hilchos Teshuvah 8:1) seems to learn that it is discussing the Olam ha'Neshamos immediately after death (which, he maintains, is the *only* place for ultimate reward).
(b) It is clear that some Neshamos do not have an Olam ha'Neshamos, as the Ramban in Vayikra (ibid.) explains with regard to someone who is Chayav Kares and has more Aveiros than Mitzvos. The Ibn Ezra learns that those who are punished with Kares "in both worlds" (Sanhedrin 90b) have absolutely no afterlife after death. On the other hand, there are those who do *not* receive Olam ha'Neshamos, but *do* merit Techiyas ha'Mesim, as Rabeinu Bachye (Bamidbar 16:33) writes regarding Korach and his group.
Of course, if a soul does not merit Olam ha'Neshamos or Olam ha'Ba, that does not mean that it is not around. It can still be around, and be punished eternally. The Gemara presents a list of sins warranting punishments of eternal Gehinom (and not just 12 months) in Rosh Hashanah 17a. An enlightening discussion of this topic can be found in Rabeinu Bachye, end of Acharei Mos. See also the Sha'ar ha'Gemul of the Ramban, at length.
In your response you wrote:
>>Of course, if a soul does not merit Olam ha'Neshamos or Olam ha'Ba, that does not mean that it is not around. It can still be around, and be punished eternally. The Gemara presents a list of sins warranting punishments of eternal Gehinom (and not just 12 months) in Rosh Hashanah 17a."<<
However, the gemara in Avodah Zara 3b says, "Ain gehinnom laasid lavo," there will be no Gehinnom in the Future to Come. It would seem from there that not even the worst souls are punished eternally.
The Gemara in Avodah Zarah continues "Rather, Hashem will take the sun from its sheath... and the wicked will be scorched by it and suffer in its rays." The punishment of the wicked clearly continues despite the fact that "there is no Gehinom."
The Ramban in Sha'ar ha'Gemul (#68-76, in the Sofer edition) discusses this Gemara at length. He first cites the Rambam, who seems to explain that Reish Lakish is suggesting (disagreement to other Ma'amarei Chazal) that Gehinom is not a specific place but a form of suffering.
The Ramban himself takes issue with this reading, and explains that there are two stages to Gehinom:
1. Gehinom that follows death immediately (i.e. in "this" world);
2. Gehinom of the World To Come, which begins after the Great Day of Judgment.
Reish Lakish is discussing the second Gehinom; all agree that the first is indeed a "place" of suffering by being burnt in a "Heavenly Fire."
The Ramban adds (#76) that the suffering at the hands of the "unsheathed sun" is indeed akin to the suffering of the first Gehinom, as it involves the same heavenly fire that burns the souls of the sinners.
May Hashem sign all of us in the book of the eternal living, that we may delight in His goodness!