Can an Oligarchy have a congress?

Yes, they sure can. A lot of third world countries with parliaments operate like that. Only the wealthy can get elected or have access to polling locations while many people in rural areas and slums don't even so much as know when elections are or who represents them. The small percentage of wealthy people then promote their own interests. Russia is like this, but that's because the already existing oligarchy will not allow anyone else to run. They'll jail them, fudge numbers, launch massive propaganda campaigns, deny challengers access to public platforms, etc.