It sounds like you have a cage setup similar to my guinea pig Lacey. I have a large play pen that she is in most of the day, then a smaller cage at night. Lacey doesn’t want to be picked up, and most guinea pigs are like that. Some tolerate it better, others may just want to hide. It takes a while for them to relax in a new place and then feel safe enough to go for a good run.
When Oreo gets in the large area, does he explore at all? Usually guinea pigs take a walk around a new area just to memorize where to run and hide from predators.
Is he eating well otherwise? Plenty of Timothy hay, some fresh veggies, and a couple of tablespoons of pellets daily. If he has a favorite food, try feeding him that one food only in the large area and make a food trail that he has to follow. He should forage by nature, so scattering food gives him something to do and will make him more inquisitive.
The tunnels and hideys should help him feel safe. Also, a piece of fabric over one corner of the play area comforting and may help him feel safe.