The Old Testament was primarily written in Biblical Hebrew. The collection contains books composed and edited over many centuries, and Hebrew is the dominant language of the Hebrew Bible, also known as the Tanakh. Its name reflects its three sections: Torah, Nevi’im, and Ketuvim.
A small but important portion of the Old Testament was written in Biblical Aramaic, especially sections of Daniel and Ezra. Aramaic became widely used across the ancient Near East, particularly during and after the Babylonian exile, which explains its presence in later biblical passages. This minority usage does not change the standard answer: Hebrew was the primary language.
Greek is another common source of confusion. The Septuagint, a Greek translation of the Hebrew scriptures, was produced in stages beginning in the third century BCE, but it was a translation rather than the primary original language of the Old Testament. Latin became important later through translations such as the Vulgate. Hebrew manuscripts were also copied and standardized over time, notably through the work of the Masoretes between roughly the sixth and tenth centuries CE.