The Ketef Hinnom text is the oldest known text of the OT (Numbers 6:24-25), dating to c. 625 BC. It is one of the only OT texts archaic enough to have been written in paleo-Hebrew orthography (aside from later archaizing texts) and it clearly has YHWH:
The next oldest texts are the Dead Sea Scrolls which represent a textual tradition distinct from both the Septuagint and the Masoretic text, and was produced by an Essene religious community (as opposed to the Pharisees). These texts frequently have 'l or 'dny for YHWH (as by the first century BC, the name had long fallen from everyday use), but many texts continue to preserve the name in archaic paleo-Hebrew script and the name also occurs in sectarian texts as well. For instance, here is part of 11QPs a, one of the scrolls of the Psalms:
Several examples of the tetragrammaton in paleo-Hebrew characters appears above. As for the Greek Septuagint, the name YHWH occurs in Hebrew characters in early surviving copies from the first century BC and the first century AD:
The earliest surviving Masoretic text was over a thousand years later.