It is also important to address the structure of the play in answering this question. Iago's soliloquys shape the entire play. If not for his soliloquys, we would not understand how his scheme is planned, how he contradicts himself each time he reveals his motives, why his ego is hurt at points during the play, and i the end, each soliloquy adeptly forwards the plot. For the audience, without the knowledge of Iago's plans, we would not entirely understand his power over Othello, or for that matter, any character in the play. The structure of the play, therefore, mirrors Iago's control.
Othello has a plot based on deception. It is based around the scheming of Iago. Iago hates Othello and so with the help of Rodrigo awakes Desdemona's father. Desdemona having just run away with Othello. Starting there, the rest of the plot continues the same way, with Iago convincing others (as he did Roderigo) or perceived threats or slights that are not actually there. Iago decides to implicate Cassio in an affair with Desdemona, to ruin Othello's life. Iago sets it up, so that Othello begins to suspect Desdemona (he finds her handkerchief in Cassio's possesion, for example). Othello, convinced by Iago of Desdemona's guilt, decides to kill her. Iago convinces Rodrigo to kill Cassio (although Iago ends up killing Cassio himself), Othello kills Desdemona, learns that he has been wrong about her affair, and kills himself, not before trying to kill Iago, who is led off alive.
As you can see, the plot is constructed through a series of deceptions, all created by Iago.