Either,
1. David. So the sense is, O Lord, preserve and assist the king, that when we are distressed and cry to him for help, he may be able and ready to help us. Or,
2. God, the supreme Monarch, the King of kings, and in a peculiar manner the King of Israel, hear and answer us, when we pray for our king and people. And for the change of persons in this verse, nothing is more common. Or,
3. Christ, called the King both in the Old and New Testament. But this verse is by divers learned men rendered thus, Lord, save the king; he (i.e. the Lord) will hear us (or, let him hear us; for the future tense is oft put imperatively) when we cry or call upon him. And this version is very agreeable to the Hebrew text. For whereas the only ground of the other translation is, that the Hebrew accent called athnach is put under the word save, which is supposed to stop the sense there, it is sufficiently evident that athnach doth not always make such a distinction in these poetical books, as appears from Psalms 11:5, Psalms 17:10 19:4 22:31, and therefore this may seem to be the better version.