We get the first indication that Othello's power isolates him when Iago speaks to Roderigo in the opening scene. Iago complains that Othello would not even listen to three prominent men of the city who were supporting Iago's promotion to lieutenant. Othello, according to Iago, dismissed them, "loving his own pride and purposes" and "horribly stuffed with epithets of war," with a curt, "I have already chose my officer." Iago suggests here that Othello is so prideful that he will not listen to the opinions of others, isolating himself with his authority.
This image of Othello is not completely sustained in the second scene, but it is clear that Othello has more power than Brabantio because the Duke is willing to hear Othello's side of the story when he finds out that it is Othello that Brabantio is accusing of stealing his daughter. After Othello's speech and Desdemona's confirmation, the Duke gives the couple his blessing, and Othello, as well as Desdemona, becomes alienated from Brabantio.
In Act 2, Othello's power as chief in command in Cyprus isolates him from his dear friend Cassio. As general, Othello must punish the drunkened Cassio who injured Montano: "I love thee, /But never more be officer of mine."
In Act 3, Othello's position as general and head of state allows him to judge, convict, and sentence Cassio and Desmona of disloyalty or, in his mind, treason. He orders the death of Cassio, "Within these three days let me hear thee say/ That Cassio's not alive," and plans to kill Desdemona himself.
Because Othello is such a prominent man, a powerful soldier, and the supreme authority in Cyprus, he becomes more and more isolated from those that are truly his friends and more and more dependent on the manipulative Iago.