The Old Testament is frequently used in the New Testament. Its authors wanted the New Testament to be seen as the fulfilment of the Old Testament and Christianity as the natural successor to Second Temple Judaism.
Additonal Answer:Depending on who is doing the counting, there are between 600 and 3,000 direct quotes, references, allegories to the Old Testament. Jesus Himself, quoted from it about 100 times. The most quoted book of Scripture is Isaiah, often referred to as the 'Little Bible.'