High oxygen producing plants

[what..the?]

I would like to know which plants produce the most oxygen? I have heard ferns are good, is this right and/or is there any others.

I want to put the plants round my pet rats cage as he has bad lungs/breathing and at present he lives in a no plant house which tends to have the windows closed all day as we are out at work.

Plants that are easy to look after would be good also.

Any ideas, thanks.