In Arrow Season 02, we found out that Oliver had caused a girl to get pregnant. His mother, Moira, had paid off the girl to leave Starling City and not tell Oliver and tell him that she lost the baby. In Arrow Season 04, Oliver found out that his child was indeed still alive, and thus made a deal to try and see him when he could. In Arrow Season 05, his son, William, was kidnapped by Adrian Chase, and used as bait to lure Oliver to Lian Yu for the season finale.
“I’m excited that not only are they going to learn about having the father/son relationship, I think we’re going to explore that in Season 6, but [William] also knows that Oliver is the Green Arrow. So there’s a lot of interesting layers and dynamics. I’m just excited to have to teach him a lesson. As a father, Oliver will definitely try to be better than what he’s been as a person… So hopefully he can be a good example for William.”
This does make one wonder what happened to William’s mother, who was also on Lian Yu in the finale. Does this mean that she’s dead? We’ll find out when Arrow returns this fall.