In original form, the Old Testament was a loose collection of stories passed down by Bronze Age Israelite nomads, slowly transcribed over the centuries and collected into texts, eventually coupled with the New Testament, earliest parts of which were the epistles of Paul, dating about 50 CE.
The oldest complete manuscript of both Old and New Testaments in existence is the Codex Sinaiticus, dating from the year 350 CE. Assuming older means closer in accuracy to the original writings, the Sinaiticus document differs from modern bibles (King James version, for example) in nearly 15,000 ways. 15,000 errors. Some of these were incidental and accidental mis-transcriptions, others were quite intentional and substantial, inserted to fit agendas du jour.
One example: The famous gospel of John 7:53-8:11 "let he without sin cast the first stone/judge not lest ye be judged" is a complete fabrication, inserted by a monk around 600 CE. It does not exist in the Sinaitcus. You can see for yourself, the entire Sinaiticus is translated and available for study online here:
Codex Sinaiticus is one of the most important books in the world. Handwritten well over 1600 years ago, the manuscript contains the Christian Bible in Greek, including the oldest complete copy of the New Testament.