I understand the small space thing. If they use one box willingly that's great! A lot of kitties get stubborn and won't share.
You should be able to find something that will work at the dollar store. They should have those dish tub things or something similar.
I would think Oreo has been dumped or let outside and wandered away. Once kitties get past that cutesy kitten stage a lot of people dump them or decide it's time they become "outdoor" cats.
Don't get the expensive vanilla when you are shopping. The cheap stuff works as well.
Good luck.