Dr. O'Connor is a member of the Brotherhood whom you first meet in Chapter 4 of the Story.
Later, he moves to Silverton. He plays an important role in the main storyline and will be involved in several story quests.
F.A.Q.
1. The Doctor asked me to help build a hospital. Where can I find the building site?
Look for a construction crate located near the Doctor. By interacting with this crate, your character will open the building interface where you need to place the required resources.
The Doctor will ask for this assistance twice during the storyline. Each time, the necessary crate will be placed nearby.
2. After Chapter 5, the Doctor is supposed to meet me at my Ranch to unlock new quests and continue the storyline, but he does not appear.
You need to complete your business in Deepwater first. Continue progressing through the secondary quests in the Marshlands until you reach the quest Revenge Served Cold. After completing this quest, you will be able to move forward.