What moral does the Chorus express about the life and downfall of Oedipus in Oedipus Rex?

You might want to consider the final words of Choragos,
the leader of the Chorus, in this excellent play. After the blinded, crushed and broken
Oedipus exits the stage, wishing himself to be exiled as he pronounced this should be
the fate of the murderer of Laius, Choragos is left to deliver the last words of the
play which also contain the moral lesson that is drawn from the life and downfall of
Oedipus:

Let
every man in mankind's frailty

Consider his last day; and
let none

Presume on his good fortune until he
find

Life, at his death, a memory without
pain.

The transformation in
the circumstances of Oedipus therefore leads to this moral lesson. The lesson is clear.
We should never count our lives as being happy or "presuming on our good fortune," until
we die, because, as the play has evidently shown, bad fortune can strike at any moment
and bring with it such a reversal of circumstances that can transform our lives
utterly.