Who will tell Hashem's straightness?
Rashi (15): The Tzadik who is fresh in his old age. Malbim - he will be fresh in order that he will be able to tell this.
Radak: The entire world, in the days of Mashi'ach. Nowadays, people's hearts waver. They see serenity of Resha'im and afflictions of Tzadikim. 'We cannot explain serenity of Resha'im, and not even afflictions of Tzadikim' (Avos 4:15). When Mashi'ach comes, people will be refined, and there will be no serenity of Resha'im or afflictions of Tzadikim, like it says in Mal'achi.
Why did he say "Tzuri"?
Radak: I trust in Him.
Who says "v'Lo Avlasah Bo", and what does it mean?
Radak: Everyone will say so. It means that all His deeds are with Emunah. Hashem will reveal why there was serenity of Resha'im and afflictions of Tzadikim in this world.
Malbim: The old Tzadik says so. There is no injustice in His conduct, even if it seems that Resha'im succeed in this world. Everything is with Hashgachah; in the future, Tzadikim will receive their reward.
Why is it written Olasah, and we pronounce it Avlasah?
Radak: They mean the same. The Kesiv is like verbs in which the middle letter of the root is sometimes silent. We find "v'Olasah Kaftzah Fiha" (Iyov 5:16), "b'Lev Olos Tif'alun" (above, 58:3). In the pronunciation, the Vov is pronounced, like it usually is.
