Return String values using LOD calculation

Hi All, how can I return string values using LOD, or any suggestions? Please Note the string field name ( [ORDER_NUMBER]) is a combination of Alphabetical and numeric characters.

in the attached image below looking at Serial number 1899, my desired results is to see highlighted order numbers returned since the ACTIVITY DATE is greater than LAS_OD_DATE

{ FIXED [SERIAL_NUMBER]: 

MIN(IF [WO_TYPE] = 'REPAIR'

AND [ACTIVITY DATE] >= [LAST_OD_DATE]

THEN [ORDER_NUMBER]

END)}