What is the meaning of "Oz Lo Vach"?
Rashi: He makes You the strength on which he relies.
Radak citing his father: He has strength and Chachmah in knowledge of Your unity, which is the root of Your Avodah, like David told Shlomo "Da Es Elokei Avicha v'Avdehu" (Divrei ha'Yamim I, 28:9). This is right after "Ashrei Yoshevei Veisecha", for those who dwell in Your house have Oz in You. They isolate themselves there, and engage in Chachmah to know Hashem.
Radak citing Ibn Ezra: This refers to Olei Regalim. Happy are those Yisraelim who celebrate [the festivals in the Mikdash]. They need Oz, for they go on the roads, and they have Oz in You.
Malbim: Even though in practice, we cannot ascend to Beis Hashem, happy is one who does not despair, rather, he strongly anticipates Hashem's salvation.
What are "Mesilos bi'Lvavam"?
Rashi: In his heart he thinks about the paths of his ways, to straighten his ways.
Radak: They have in their hearts paths of knowing You, and pass and return on those paths, and every day they increase vigor.
Radak: The only desire and thought in their hearts is for the paths that will lead to Beis Hashem.
Mesilas Yesharim (Perek 20): Three things are needed to succeed in weighing matters of Chasidus. (a) His heart is totally straight. It has no leaning other than to be pleasing to Hashem. (b) He greatly investigates his deeds, and tries to fix them according to this goal. (c) After all this, he casts his load on (trusts in) Hashem.
Malbim: In their hearts they find paved roads via which they will return to Tziyon. I.e. even though there are not actual roads, they are in their hearts. They are full of hope in Hashem's strength.
