You will first meet Dr. O'Connor in Chapter 4 of the Story during your introduction to the Brotherhood. Find common ground with him, and he will surely help you. Later on, Dr. O'Connor will move to Silverton, where he will need your help again to rebuild his home and medical office. In return, he will help you become a bit healthier!
Eventually, he will be the one who will ask you for help as an old friend when the time comes to go to the aid of the people of Teklanika!