Here the word "Oh" is used to convey a strong desire for people to praise Yahweh. The abstract noun "faithfulness" can be translated with an adjective. AT: "Let people praise Yahweh because he is faithful to his covenant" or "People should praise Yahweh because he is faithful to his covenant" (See: [[rc://en/ta/man/translate/figs-exclamations]] and [[rc://en/ta/man/translate/figs-abstractnouns]])
It is implied that these are people that Yahweh has rescued. AT: "Yahweh also rescued people who sat" (See: [[rc://en/ta/man/translate/figs-explicit]])
Both "darkness" and "gloom" have basically the same meaning and are used to emphasize how dark the prison was. AT: "in complete darkness" (See: [[rc://en/ta/man/translate/figs-doublet]])