While there is some disagreement between different Christian traditions regarding the extent of the Old Testament, these disagreements should be understood within the context of broad agreement overall.
To begin with, the three main branches of Christianity (Orthodox, Catholic, and Protestant) as well as rabbinic Judaism all agree the Old Testament includes the core Hebrew texts, commonly known as: Genesis, Exodus, Leviticus, Numbers, Deuteronomy, Joshua, Judges, Ruth, 1 Samuel, 2 Samuel, 1 Kings, 2 Kings, 1 Chronicles, 2 Chronicles, Nehemiah, Esther, Job, Psalms, Proverbs, Ecclesiastes, Song of Solomon, Isaiah, Jeremiah, Lamentations, Ezekiel, Hosea, Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, and Malachi.
There is also universal agreement on the inclusion of the Aramaic texts Ezra and Daniel within the Old Testament.
The texts which are disputed are primarily Greek, and that in itself should give us a hint as to why they’re disputed. Quite simply, they were composed later than the core Hebrew texts. In general, the Orthodox and Catholic traditions of Christianity tend to include them whilst Judaism and the Protestant tradition of Christianity tend to exclude them. The texts in question are 2 Maccabees, 3 Maccabees, 4 Maccabees, 3 Esdras, Prayer of Manasseh, Wisdom, the Letter of Jeremiah from Baruch, and the additions to Daniel known as Prayer of Azariah, Susanna, and Bel and the Dragon. There are also a few Hebrew texts (1 Esdras, Judith, 1 Maccabees, Baruch, Sirach) and one Aramaic text (Tobit) which are also disputed, but by and large it is for similar reasons, due to later composition. It is worth noting however that few doctrines of any of the above traditions are based solely on these disputed texts. Their impact should not, therefore, be overstated.