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).
@sandip_mep what simulation tool are you using? Please mention it in the title or body of your post, as well as add a tag so that others can provide better help.