When we use the language “Old Testament,” we’re referring to the thirty-nine books that run from Genesis to Malachi. Occasionally a Bible reader will push back on the nomenclature, arguing that Christians should call those books the “Hebrew Bible” or the “Hebrew Scriptures” or the “Tanak” or the “First Testament” or the “Jewish Bible” instead of the “Old Testament.”
I'm surprised that you didn't reference Hebrews 8:13.