News of Slade's return from the Phantom Zone caused Clark to head there in hopes of determining who opened the door. Oliver managed to tag along against Clark's wishes, where they encountered Zod who had been sent there by the Kandorians.
Darkseid had come to looking for Zod and offered Zod rulership over the Zone if he would kill Clark. Knowing Oliver had been marked by Darkseid, Zod appealed to the darkness within him and pitted he and Clark against each other in a gladiator style fight to the death.
Oliver fought Clark aggressively and after a bloody row ran a sword through Clark. When Zod was convinced of Oliver's allegiance, Oliver ripped the crystal to the Zone gate away from Zod and Clark activated it taking them home and destroyed the crystal in the process, blocking any future reentry.
While they had Clark and Tess planned to destroy the gate to the Zone permanently, whether they had returned or not. When Lois discovered the self destruct sequence, she forced Tess at gunpoint to cancel it.
After Clark said that any one of the Leaguers who might be infected by the darkness would have to be locked up, Oliver decided that he needed to go on his own after the weapon Hawkman once said they had used to defeat Darkseid.