Going into any reading or viewing of Oedipus Rex, the audience would know that:

• Oedipus has already killed his father
• married his mother (committed incest)
• fathered four children who are also his siblings

Oedipus is a universally known myth, and all of the background information (or antecedent action) is common knowledge.  I don't know if those count or not, since they have taken place before the play begins.

During the play, we (the audience) know more than any of the characters on stage, except for Tiresias.  The play is driven by dramatic irony, and here are the main ironies of part I only:

• We know the cause of the plague in Thebes: incest and murder
• We know Oedipus' curse on the murderer of Laius will lead to his banishment
• We know that Oedipus' search for the murderer will lead to his knowledge of his own crimes
• We know that Creon is telling the truth, that he's not a traitor
• We know that Tiresias is telling the truth, that Oedipus is the murderer
• We know that Jocasta will know the truth sooner than Oedipus, and her suicide will finally confirm the truth
• We know that Oedipus will gouge his eyes, an ironic punishment for not knowing or seeing the truth

The effects lead to katharsis (purgation of pity and fear) in the audience.  We pity Oedipus for seeking to know the truth only to have that truth backfire on him and make him suffer.  We fear that his plight might be our own: not that we will commit his two crimes necessarily, but that we may never know the truths about ourselves and our families, and even when we do, it leads only to pain and suffering.