Chase has Oliver for six days. By the second scene in the cell that time is already starting to wear on Oliver. The entire scene he talks as if he’s slightly out of breath, despite sitting still and having no obvious injuries. He’s sleep deprived, certainly- it’s hard to sleep when you’re chained to a concrete floor awaiting torture at the hands of a madman. Not to mention the fact that the time spent with Chase is probably during the night since at this point Chase is still keeping up appearances as DA, so Oliver’s circadian rhythms and internal clock are being disrupted. We’re also never shown Oliver being given food or water. In six days he must have been given water at some point, or he’d die. Food... Probably some, since when he is released he’s strong enough to make it back to the bunker. But I would bet that whatever Chase gives him to eat, it’s not enough. And he’s stuck shirtless, on plain concrete.
So besides all the torture we see, Oliver spends six days being sleep-deprived, starving, probably cold, chains pulling at his wrists, and in constantly increasing amounts of pain. And then manages to drag himself home without being seen and stitch himself up. Because he’s amazing.