Oliver Sullivan is the father of Cassie and Sam Sullivan. He is quite an intellectual man and always tries to be the voice of reason. He had many intelligent sounding theories on The Others before the 1st Wave. After each of the waves, he begins to understand their motives better.
He lost his wife in the 3rd Wave leaving him to take care of their daughter and son. He was killed during the fourth wave at Camp Ashpit. His two children managed to survive most of the waves but ultimately, only his son Sam lived through The Others' invasion.
He is portrayed in the film adaption by Ron Livingston.
As a boy, Oliver went on a trip to a planetarium where he lost himself at the display of stars. He especially was taken by the constellations. The one that really had his eye was the constellation of Cassiopeia. Fifteen years from that moment, he would come to name his daughter after that constellation.The Others when they first arrived. He and his wife decided to stay home and try to keep going about their lives like normal unlike a few other people around them were doing (running away, packing everything up, etc.) He explained that they didn't feel safe being in such densely populated towns but reasoned that they were as safe as anywhere for the time being. At first, he debated with Cassie that the Others might be there to 'save' them. When the 1st wave hit ten days after the Arrival, he went to Cassie's school to pick her up. They had to walk all the way back home through all the chaos that the EMP caused in the streets. Oliver was quickly able to identify what happened as an EMP as he took Cassie home. With how chaotic things had become he felt that the wise choice was to hold up at home even after news reached them of the 2nd wave hitting the coastal cities. When the 3rd wave hit, Oliver's wife was infected. However, it seemed that he and their children were immune to the disease. He stayed with his wife until the end and then gave her a burial in their backyard. When that was done he took his children and made to travel to Wright-Patterson. He felt that the base might be the safest place for them. On the way there they stumbled onto Camp Ashpit and decided to stay there a while with the other gathered survivors. He became concerned when the drones started to be sighted and guessed that the Others were using them to test conditions for themselves to finally make the Earth their home. He, like the rest of the camp, was shocked when soldiers pulled up with working vehicles and buses. Their leader Colonel Vosch explained that they had been in quarantine and now were seeking survivors. They were taking the children as the first priority as he claimed they couldn't be sure that they were carriers to the plague. When his son Sammy was taken Vosch called for the rest of the camp to gather together. That didn't sit right with Oliver and when his daughter Cassie noticed that one from their camp was missing he insisted that she go and look for him.
His worries were proven right when the soldiers opened fire on them revealing themselves as the 4th wave. He was wounded and crawled through the chaos. He spotted Cassie watching it and silently told her to run away before he collapsed. Vosch then came by and shot him in the head effectively killing him.
Cassie begins to tell Sam about how their father had been killed though Sam guessed that their dad had to have died some time ago. At first, Sam was mad at both Cassie and their father but Sam calmed down and then wanted to know how he died. Cassie spun a story of how their father had fought hard against Vosch and the other Silencers before he finally was brought down by him. Sam vowed that he would get Vosch and avenge their father but Cassie told him that she already had dibs.
In the film, Oliver met his fate when Vosch and his soldiers opened fire on the gathered people of Ashpit. Cassie had returned to find his dead body lying with the other corpses.