You could use the OnObjectEnter trigger to fire this script:
Set Field ["no field name here" ; If ( IsEmpty ( Get ( ActiveFieldContents ) ) ; "OK" ) ]
Commit Records/Requests [ ]
This script works for every active field ( whenever you click on a field with that trigger )
But you might also be able to not use any script at all if you format the field with a single check box for the value "OK".