The Old Testament (abbreviated OT) is the first part of Christian Bibles, based primarily upon the Hebrew Bible (or Tanakh), a collection of ancient religious writings by the Israelites believed by most Christians and religious Jews to be the sacred Word of God. The second part of the Christian Bible is the New Testament. The books that comprise the Old Testament canon, as well as their order and names, differ between Christian denominations.
Quran confirms the True statements mentioned in the Old and New Testaments, but since many parts of the Old and New testaments were been distorted by people, the real testaments are not what is been claimed.
Quran does not confirm every thing which is been claimed by People of Book but it definitely confirms the Real facts in the original Testaments. This confirmation is mentioned in many Suras like:
al-Baqara 41, 91, 97,
Aal Imraan 3,
al-Ma'ida 46, 48
The story of the monk called Bahira is not considered authentic in the Imami school.