Why did he say "Hashem Oz Lamo"?
Radak: (a) Since David sat in Beis Hashem, he prayed that Yisrael have Oz (strength) when they go to war. Their hearts should not be soft when he is not with them in war. (b) Hashem should give to them strength in the war to overcome their enemies.
Malbim: Refer to 28:8:2:2.
What is the meaning of "u'Ma'oz Yeshu'os Meshicho Hu"?
Rashi: Refer to 28:7:4:1. Radak - "Meshicho" is David himself, like "Meshi'ach Elokei Yakov" (Shmuel II, 23:1). The prefix Vov in u'Ma'oz means Ka'asher (when), like "Im Lo Yisbe'u va'Yalinu", "v'Sifsei Renanos Yehalel Pi" (59:16, 63:6).
Malbim: Since Hashem made David king over Yisrael, his salvation is tied to Yisrael's. Since Hashem is Yisrael's strength, He is the protection of His Mashi'ach; He must save in the merit of Yisrael.
