Just asking...every pic I see of a oai flapper door the flapper is are always open without vacume to it.When installed just how does it work?

 

Vacuum closes the door at high vacuum, and the motor pulls warm air when the door is closed.
When vacuum drops, the door opens and the carb gets outside air - hence 'outside air induction'.

 

To expand on Rickman's simple explanation:

Engine shut off, there is no vacuum. With no vacuum, flapper defaults OPEN.

Start engine, there's vacuum. Flapper closes and stays closed under normal cruise conditions where vacuum is high.

Floor the throttle, vacuum drops off and flapper opens, allowing cold (well, ambient temp, still cooler than underhood air) outside air into the aircleaner.