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You need to use EMS for the control.
Here is an example code for your case. Replace the "VAV_System_Default OA Controller" with the name of your "Controller:OutdoorAir". And change the xxx, yyy, zzz to your control parameter.

EnergyManagementSystem:Actuator,
VAV_System_Default_OA_Flow_Rate,    !- Name
VAV_System_Default OA Controller,    !- Actuated Component Unique Name
Outdoor Air Controller,    !- Actuated Component Type
Air Mass Flow Rate;        !- Actuated Component Control Type

EnergyManagementSystem:Program,
Set_VAV_System_Default_OA_Flow_Rate,    !- Name
IF totalAirFlow < xxx,    !- Code, xxx is your changing point in kg/s
Set VAV_System_Default_OA_Flow_Rate = yyy,    !- Code,  yyy is your minimum OA flow rate
ELSE
Set VAV_System_Default_OA_Flow_Rate = totalAirFlow * zzz, !- Code, zzz if your proportional ratio
ENDIF;;                    !- Code

EnergyManagementSystem:Sensor,
totalAirFlow,
VAV_System_Default OA Controller,
Air System Mixed Air Mass Flow Rate; 

EnergyManagementSystem:ProgramCallingManager,
Control_VAV_System_Default_OA_Flow_Rate,    !- Name
AfterPredictorAfterHVACManagers,    !- EnergyPlus Model Calling Point
Set_VAV_System_Default_OA_Flow_Rate;    !- Program Name 1

You need to use EMS for the control.
Here is an example code for your case. Replace the "VAV_System_Default OA Controller" with the name of your "Controller:OutdoorAir". And change the xxx, yyy, zzz to your control parameter.

EnergyManagementSystem:Actuator,
VAV_System_Default_OA_Flow_Rate,    !- Name
VAV_System_Default OA Controller,    !- Actuated Component Unique Name
Outdoor Air Controller,    !- Actuated Component Type
Air Mass Flow Rate;        !- Actuated Component Control Type

EnergyManagementSystem:Program,
Set_VAV_System_Default_OA_Flow_Rate,    !- Name
IF totalAirFlow < xxx,    !- Code, xxx is your changing point in kg/s
Set VAV_System_Default_OA_Flow_Rate = yyy,    !- Code,  yyy is your minimum OA flow rate
ELSE
ELSE,
Set VAV_System_Default_OA_Flow_Rate = totalAirFlow * zzz, !- Code, zzz if your proportional ratio
ENDIF;; ENDIF;                    !- Code

EnergyManagementSystem:Sensor,
totalAirFlow,
VAV_System_Default OA Controller,
Air System Mixed Air Mass Flow Rate; 

EnergyManagementSystem:ProgramCallingManager,
Control_VAV_System_Default_OA_Flow_Rate,    !- Name
AfterPredictorAfterHVACManagers,    !- EnergyPlus Model Calling Point
Set_VAV_System_Default_OA_Flow_Rate;    !- Program Name 1

You need to use EMS for the control.
Here is an example code for your case. Replace the "VAV_System_Default OA Controller" with the name of your "Controller:OutdoorAir". And change the xxx, yyy, zzz to your control parameter.

EnergyManagementSystem:Actuator,
VAV_System_Default_OA_Flow_Rate,    !- Name
VAV_System_Default OA Controller,    !- Actuated Component Unique Name
Outdoor Air Controller,    !- Actuated Component Type
Air Mass Flow Rate;        !- Actuated Component Control Type

EnergyManagementSystem:Program,
Set_VAV_System_Default_OA_Flow_Rate,    !- Name
IF totalAirFlow < xxx,    !- Code,  xxx is your changing point of the total flow rate in kg/s
Set VAV_System_Default_OA_Flow_Rate = yyy,    !- Code,  yyy is your minimum OA flow rate
rate in kg/s
ELSE,
Set VAV_System_Default_OA_Flow_Rate = totalAirFlow * zzz, !- Code, zzz if is your proportional ratio
ENDIF;                    !- Code
End of code

EnergyManagementSystem:Sensor,
totalAirFlow,
VAV_System_Default OA Controller,
Air System Mixed Air Mass Flow Rate; 

EnergyManagementSystem:ProgramCallingManager,
Control_VAV_System_Default_OA_Flow_Rate,    !- Name
AfterPredictorAfterHVACManagers,    !- EnergyPlus Model Calling Point
Set_VAV_System_Default_OA_Flow_Rate;    !- Program Name 1