The power relations in Shakespeare’s Othello are complex.
The most powerful character in the play is the Duke of Venice, who is in the greatest position of authority. It is his job as Duke to give military orders and make decisions.
Brabantio tries to command Desdemona not to marry Othello. He has the right to make this decision because he is her father. Some of the reason is prejudice, some worry that the marriage will not turn out well. Othello and Desdemona outwit him.
Iago is jealous of Othello and Cassio, and therefore plots their downfall. He doesn’t really have official power over them (Othello is his boss) but Othello (mistakenly) trusted him and lets himself be influenced by him.