The most obvious are what we would call quotations. Scholars give different estimates here, but by my estimate Paul quotes the OT about ninety-two times in his thirteen epistles. It’s worth noting that those quotations are not evenly distributed throughout, which is kind of an interesting phenomenon. Some of the letters, like Romans, are full of quotations; others have none at all. [It’s] an interesting phenomenon, again, that we’ll have occasion to talk about later on.