I wouldn't rank Oscar's left hook to the head as the best at 147 at that time, that honor would have to go to Tito. However, Oscar's left to the head was better than everyone else besides Tito, and his left hook to the body was the best in the division.
Quartey was a somewhat robotic fighter. He stood straight up, didn't bob and weave, and threw that great jab followed by right hands.
In the DLH fight, he was very effective with the right hand, buckling DLH several times.. The problem is, he wasn't good at bringing the right back quickly, so when he threw a right hand, it would hang out there and he was vulnerable to counter hooks.
Upright, mechanical fighters have to bring their hands back quickly, or they'll always be vulnerable to counters upstairs.