The Peshitta Old Testament, unlike the Peshitta New Testament, is a translation of the original Hebrew Bible. What is especially interesting and valuable about the Peshitta Old Testament is that it is the oldest Semitic version of the complete Old Testament extant. It is commonly believed by scholars to be a 2nd century AD translation. I believe that it is older than that, based on its readings and also on many of the headings of the Psalms in Eastern Codex Ambrosianus of the 6th century, several of which actually date the original text in the 1st century A.D.
The text of The Peshitta Psalms generally follows the Massoretic Hebrew Bible text, though with quite a number of interesting differences. Some of these are due, no doubt, to differences in the Hebrew text which was originally translated 2000 years ago; others are translational variations of particular Hebrew words in Aramaic; some seem to be Christian-Messianic interpretations of certain Psalms, which demonstrate the Christian theological viewpoint of the translator(s). Vocabulary and syntax seem generally akin to the New Testament Peshitta, as if the translators were familiar with the Aramaic Peshitta NT. Proverbs generally follows a text like that followed by the Greek LXX (Septuagint). Some Dead Dea Scrolls represent this kind of text of 2100 years ago and the New Testament quotations of the Hebrew Bible seem to have a sort of LXX-Massoretic hybrid text as their source. The text type varies from book to book of the O.T.