There is no fresh air entering the aircraft without packs ON. It does not enter through any Fwd Outflow Valve, Flow Ctrl Valve or OEV.
The venitlation is from the differential pressure which is createdfrom the fully open outflow valve.
If you look at the B737 MEL for Unpresurized flight, it mentions ventilation by the OFV:
5. The following steps can be taken to improve both passenger-cabin and crew-compartment temperatures:
A. If possible, pre-condition the cabin to a low temperature prior to dispatch. A low initial cabin temperature will result
in a lower peak temperature.
B. Maintain the highest allowable flight altitude.
C. Minimize cabin heat by pulling the shades down and turning off unnecessary lights.
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D. Position Gasper Fan/Recirculation Fan switches as follows:
-300/-500/-700/-800: Recirculation Fan(s) – AUTO.
E. Because of reduced ventilation rate (approximately 1/6 of normal), consideration should be given to flight duration
and passenger load so that excessive cabin temperatures may be avoided.