Here David describes Yahweh speaking as sending his words as if they were a messenger. Possible meanings are 1) "He commanded for them to be healed and they were healed" or 2) "He encouraged them and healed them" (See: [[rc://en/ta/man/translate/figs-personification]])
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. See how you translated this in [Psalms 107:8](./008.md). 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]])