Quintana improved to 25-1 with 19 knockouts. Williams, from Aiken, dropped to 33-1 with 24 KOs.
There were no knockdowns in the fight.
Quintana won with scores of 115-113, 116-112 and 116-112.
Quintana won three of the first four rounds. But Williams, who held a 5-inch height advantage, pulled even after eight rounds.
Williams was sluggish after suffering cuts over both eyes during the later rounds.
Quintana was able to take advantage. He rocked Williams repeatedly with his left hand.
Williams rallied in the final round, but it was too late.
It was the first defense of the WBO title that Williams won in July with a victory over Mexico's Antonio Margarito.