What are the conditions for the "is one of" operator being enabled for filters on a particular field?
'Is one of' operator in views filter aka ORoperator in SQL, it is only available for list fields or fields with allowed values that means any field which has the select list, radio buttons, or checkboxes widget can have and use 'Is one of' operator in views filter.

Theoretically, any reference field or any list field should have 'Is one of' operator available in views filter.