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You can set the minimum OA in the Minimum Outdoor Air Flow Rate field. If you ever want the OA to go below that value or to 0 then you can use a schedule in the Minimum Outdoor Air Schedule Name field to control when the OA falls below the minimum OA. I believe this works with or without economizer since it acts on the minimum value.

Controller:OutdoorAir ,
  OA Controller 1, !- Name
  Relief Air Outlet Node , !- Relief Air Outlet Node Name
  Outdoor Air Mixer Inlet Node , !- Return Air Node Name
  Mixed Air Node , !- Mixed Air Node Name
  Outdoor Air Inlet Node , !- Actuator Node Name
  0.8, !- Minimum Outdoor Air Flow Rate {m3/s}
  1.6, !- Maximum Outdoor Air Flow Rate {m3/s}

  MinOAFractionSched , !- Minimum Outdoor Air Schedule Name

Schedule:Compact,
  MinOAFractionSched,   !- Name
  Fraction,                    !- Schedule Type Limits Name
  Through: 12/31,          !- Field 1
  For: Weekdays SummerDesignDay WinterDesignDay, !- Field 2
  Until: 9:00,0.0,         !- Field 3
  Until: 12:00,1.0,        !- Field 5
  Until: 13:00,0.8,        !- Field 7
  Until: 14:00,0.4,        !- Field 9
  Until: 15:00,0.8,        !- Field 11
  Until: 17:00,1.0,        !- Field 13
  Until: 24:00,0.0,        !- Field 15
  For: AllOtherDays,       !- Field 17
  Until: 24:00,0.0;        !- Field 18

1.51.3.1.17 Field: Minimum Outdoor Air Schedule Name The name of a schedule which uses decimal values (e.g., 0.0 or 1.0). These values are multiplied by the minimum outdoor air flow rate. This schedule is useful for reducing the outdoor air flow rate to zero during unoccupied or start up hours. If this field is not entered, the minimum outdoor air flow rate either remains constant during the simulation period (Minimum Outdoor Air Control Type = FixedMinimum) or varies in proportion to the supply air flow rate (Minimum Outdoor Air Control Type = ProportionalMinimum).