The OAK is really about increasing the efficiency.
The DP of a window or door is not limitless... it really doesn't matter who or how they are installed... once the DP is struck... they will leak.
The OAK just keeps from using the already heated air to feed the combustion process and run up the flue. That was a major downer for open fireplaces.
Basically the fireplace would act like an air pump moving heated air outside as fast as possible... and using make up air from leaks in the shell as they reached their DP.
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