The OT was written in the Hebrew language which is now a dead language, as in no-where speaks it anymore. The Hebrew bible was found preserved in the Dead Sea Scrolls so we have a copy of many parts of the bible in its original language.
However, there are a few exceptions:
Hebrew is used in Daniel 1:1 to Daniel 2:4a and Chapters 8 through 12, Aramaic is used from Daniel 2:4a to 7:28. The Aramaic langue was the common language or lingua franca, used in Assyrian, Babylonian and Persian communication. Aramaic is found also in the Ezra 4:8-6:18, 7:12-26 and Jeremiah 10:11.
Also it is important to note that after the Jews were exiled from Jerusalem they started losing their use of the Hebrew language, and after a time very few spoke it or understood it at all. This is when the Septuagint translation was produced which was in the universal language of the time, Kione Greek. Often Jesus quotes from this version in the gospels which is why there is a slight variation in the wording between the old testament and the new when Jesus quotes it.