The OBS preview window is just a preview of your scenes at max quality, and not what the viewers will be seeing post-encoding.
Your lighting is, in my opinion, the major culprit, since it is a lot of even shades of light browns and dark browns. The closer the shades are to eachother, the more the encoder will want to just blend them together. Plus the details in the game appear to be taking more of the encoder performance to keep the visuals crisp.
Adjust your lighting more, possibly getting a light that aims at you but now the rest of the room, maybe even a back light that aims at your back. You can get clamp-on dome lights for relativly cheap at places like Home Depot, and some nice "cool white" LED lights that wont generate much heat (if any) to help in those areas. The room i am in right now has 5 LED lights (specifically GE Bright Stiks), three of them are far away from me adding general ambience, one is slightly above me overhead (all four of those are actually my ceiling lighting) and then I have a dome light aiming directly at my green screen to brighten it up.
Even if you dont have a green screen, you want to brighten yourself up so you are more prominent in the camera than the rest of your room behind you. Adjusting the white balance of the camera so everything doesnt have an amber color would also help.