Jocasta plays a very positive role in the play. We especially see her play a positive role in the way in which she tries to make peace between Oedipus and Creon, and also in the way that, like a mother, she continues to protect Oedipus from discovering the truth of his identity.
When Oedipus accuses Creon of treason and threatens to kill him, Jocasta intervenes and tries to make peace. She chastises them for their quarrel and shames them for fighting at a time when real suffering is taking place throughout the city, as we see in her lines:
Why, unhappy men, do you stir up this unwelcome of the tongue? Aren't you ashamed to stir up private evils when the land is so sick? (664-667)
She even insists that Oedipus believe Creon when he swears by the gods that he would rather die than do what Oedipus has accused him of.
In addition, Jocasta acts as a positive force in the play by trying to act as Oedipus's protector. All throughout the play we see her trying to protect him from believing in the prophecy and from discovering his true identity. At first, believing herself that her son was dead, she tries to persuade him to forget the prophecy because it couldn't possibly be fulfilled if her son was already dead. However, a messenger from Corinth comes delivering the news of Polybus's death and informs them that Polybus is actually not Oedipus's true father but instead that Oedipus was given to Polybus by the messenger himself who was given Oedipus by a shepherd. When Jocasta hears this, she finally begins to realize the truth of the prophecy and vehemently tries to dissuade Oedipus from seeking the truth further, as we see in her passionate lines, "No, by the gods! If indeed you care for your own life, do not go after this! I grieve enough" (1087-88).
Hence, we see that due to Jocasta's protective and peacemaking nature, Jocasta acts as a positive force in the play.