If you have Remote Configuration mod installed and you open the oil derek GUI from the map view, then you won't be able to see its inventory like you normally do.

Curious... Remote Configuration is one big hack so I guess I shouldn't be too surprised that it breaks something. I haven't tried it myself but I wonder if it is actually a base game bug. Cargo Ships just creates a relative GUI that is permanently there, attached to "oil_rig" GUIs. Maybe these relative GUIs aren't shown when a mod opens the entity's GUI with player.opened = entity, in which case it would be a base game bug. I'll have to experiment :)