Albert Barnes Commentary Psalms 89:20

"I have found David my servant; With my holy oil have I anointed him:" — Psalms 89:20 (ASV)

I have found David my servant - That is, I found him among the sheepfolds; in humble life. I saw there one who was qualified for the high office of being the ruler of the nation, and I designated, or set him apart, for that office. The idea is, that there was in him a precious qualification for this work, and that God had seen this, and, in accordance with this, had summoned him to his service.

With my holy oil have I anointed him - By the hand of Samuel (1 Samuel 16:13). Oil was used in setting apart prophets, priests, and kings. It was poured upon the person—emblematic of the pouring out upon him of wisdom and grace from on high to qualify him for his office.