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You can limit the maximum OA fraction using the "Maximum Fraction of Outdoor Air Schedule" in the Controller:OutdoorAir object attached to the AirLoopHVAC:OutdoorAirSystem on the Airloop.

Here is what this looks like in an OpenStudio Measure:

# Find the AirLoopHVAC:OutdoorAirSystem on the Airloop
# Find the Controller:OutdoorAir object on the OA system
# Set the Maximum Fraction of Outdoor Air Schedule to one
# that has night-time maximums set to the desired fraction.
air_loop.supplyComponents.each do |supply_component|

  # Skip any supply component that isn't an AirLoopHVAC:OutdoorAirSystem
  next if supply_component.to_AirLoopHVACOutdoorAirSystem.empty?

  # Convert the supply component to a Controller:OutdoorAir
  oa_system = supply_component.to_AirLoopHVACOutdoorAirSystem.get

  # Get the OA controller that belongs to this OA system
  controller_oa = oa_system.getControllerOutdoorAir

  # Set the Maximum Fraction of Outdoor Air Schedule
  controller_oa.setMaximumFractionofOutdoorAirSchedule(max_oa_frac_sch)

  # Tell the user what happened
  runner.registerInfo("Set the max OA fraction schedule for #{air_loop.name} to #{max_oa_frac_sch.name}.")

end