Irony is embedded into Othello right from the start of the play. In one of the earliest scenes, when Brabantio warns Othello that Desdemona may end up deceiving him, Othello gives a reply which proves to be ironic: “My life upon her faith!” The situational irony here is revealed in this line's double meaning.
While Othello means what he says in a figurative way, meaning that he has complete trust in Desdemona, it is the literal meaning of this line that will come to be true. It is Othello’s realization of Desdemona’s steadfast faith, and the consequent revelation of the horror of his own acts, that will ultimately cause Othello to take his own life. Othello’s life thus will rest upon Desdemona’s faith, but not in the way he means. The irony of the double meaning of this line also highlights the duplicity of language, with words' ability to deceive being a key part of Othello's downfall.
Accordingly, Othello's speech is also an example of foreshadowing, with his words proving strangely prophetic. At this point the audience does not yet know too much about how the play will end, but the use of foreshadowing provides the audience with hints of the tragedy that is to come.
Foreshadowings of the play’s tragic ending can be found multiple times in Othello’s speech. Before he is fully convinced of Desdemona’s alleged infidelity, Othello exclaims:
Excellent wretch! Perdition catch my soul
But I do love thee! And when I love thee not,
Chaos is come again
The last part of this speech, when Othello says Chaos will come again when he loses his love for Desdemona, foreshadows the play’s ending, when chaos and mass tragedy will ensue as a result of Othello’s turn to hatred.
The effectiveness of such foreshadowing as a device is strengthened through the simultaneous use of dramatic irony. Though Othello is unaware how prophetic his words will prove, the audience at this point has already been privy to Iago’s private plottings and knows that he plans to tear Othello and Desdemona apart. This dramatic irony ensures that the audience can spot when Othello’s words are foreshadowing the play’s ending and heightens the play’s dramatic power. Othello’s obvious ignorance to the ending that will befall him elevates the tragedy of the play and creates sympathy for him.
That the audience knows more than Othello also complicates the audience’s relation to the play, as they feel forced to become part of Iago’s plotting yet helpless to stop it. This feeling of helplessness reinforces the exploration of the idea of fate in the play, with the use of foreshadowing presenting the play’s tragic ending as inevitable and predetermined. Othello’s speech has a prophetic quality, even if Othello himself is not aware of it. That what he says will come to pass potentially alludes to the idea that language is determinative. Such an idea proves central to the play, not only in it being Iago’s mastery of language that determines the characters’ actions, but also in a broader, meta-textual way. The actions of the play have, of course, been predetermined by Shakespeare himself. The use of foreshadowing reminds the audience that the play has been carefully and deliberately constructed, an awareness that reminds the audience of the power of language.