'Oliver' in a high-pitched tone meant William was upset with him. Much like mothers everywhere called their children by their full names when shit really hit the fan, William resorted to 'Oliver' when he found himself really put out. Like when he wasn't allowed to watch an R-rated movie. Or when he couldn't stay over at the house of friend whose parents Oliver had yet to meet. Or that time he couldn't go to a movie one town over with only a friend's older, shady looking brother as a chaperone. If Oliver had been his stepdad, these would have been the times where William would have yelled 'You're not my real dad! You can't tell me what to do!' But unfortunately for William and fortunately for Oliver, neither of those statements were true. So, when William used his name like an epithet, Oliver forced himself to stay calm and always tried to remember how lucky he'd felt the moment he'd found out William existed.
'Dad' was the golden spot. It meant everything was going smoothly. 'Dad, guess what?' had quickly become one of Oliver's favorite phrases. William still didn't use it consistently, but when he did, Oliver couldn't help but smile because it meant that William was comfortable and no longer trapped in the initial awkwardness of getting to know Oliver. The first few times had been halting, William just testing out the sound and feel. But when Oliver simply responded as though it was the most natural thing in the world, William had slowly began to grow into the habit.
'Daddy' was the big gun. And much like china plates and the good silver, it was used sparingly and only for special occasions. William mostly used it for asking for things he knew he might not get, which sometimes then landed them right back into 'Oliver' territory, but sometimes it worked. 'Daddy, can I stay up with you? I like hanging out with you' or 'Daddy, can since it's a half day at school can I just spend the whole day with you at work?" Obvious manipulations, but Oliver was a sucker and William was his soft spot.
Maybe it was because Oliver wore so many hats—Mayor Queen, Green Arrow, Oliver Queen—but William had obviously already learned the power of names.