He views Oz as his own son. He took care of him and his sister since they were young. He cares deeply for Oz since he knows what Oz is going through and he hopes for happiness and joy for him. He is shown to often let Oz do what he wants and is often trying his best to cheer Oz up from time to time when he can.
He cares for Ada like his own daughter. Although it seems that he cares for her a bit too much. Like the time when he received a letter saying that she has found someone that she likes, he tore the letter in half and went crazy saying that he won't let that happen.
He views Gilbert like his son as well. Although Gil is often afraid of him than the others. Gil is often shown to be rather speechless or is often afraid that he will do harm to his master(Oz)'s uncle.
Oscar and Xai seemed to have a large strain in their relationship. Oscar did not approve of Xai's actions and the way he treated Oz and Ada. He seemed to always have some sort of suspicion and/or distrust towards his brother, though Oscar's last words were that he wished that they would've had a chance to get to know each other. However, in the end, it was Xai who shot Oscar dead
Sara Vessalius was Oscar's wife. They met during one of Oscar's frequent Clandestine visits to an old part of the town. They fell in love and got married. However, their marriage was frowned upon, but they appeared to be happy with each other anyways. When his wife dies, Oscar deeply grieved for her and feels "tortured" when Xai revealed that Jack killed his son and wife, including Oscar's. After Oscar died he could be seen with Sara and their miscarried son in the Afterlife.