CREATE TABLE TEST1
(
  OFFICE_PRODUCTS     NUMBER,
  OFFICE_ELECTRONICS  NUMBER
)

Insert into TEST1 (OFFICE_PRODUCTS, OFFICE_ELECTRONICS) Values(1, 0);
COMMIT;

CREATE TABLE TEST2
(
  EXPORT_FIELD_NAME         VARCHAR2(100 BYTE),
  EXPORT_COLUMN_EXPRESSION  VARCHAR2(100 BYTE)
)

Insert into TEST2
   (EXPORT_FIELD_NAME, EXPORT_COLUMN_EXPRESSION)
 Values ('A1', 'least(OFFICE_PRODUCTS, OFFICE_ELECTRONICS)');
COMMIT; 

select (select EXPORT_COLUMN_EXPRESSION from test2 where EXPORT_FIELD_NAME='A1') FROM TEST1; 

CREATE OR REPLACE FUNCTION test1x RETURN NUMBER
is
 a varchar2(1000);
 b number;
BEGIN
      SELECT  export_column_expression
        into  a  
        FROM  test2;
     EXECUTE IMMEDIATE ' SELECT ' || a || ' FROM test1 ' into b ;               
     return b;  
END;

select test1x from dual

CREATE OR REPLACE function fun return VARCHAR2 as
v_sql VARCHAR2(100);  
begin
execute immediate 'select  (select EXPORT_COLUMN_EXPRESSION from test2 where EXPORT_FIELD_NAME=''A1'') FROM TEST1' INTO v_sql;
return v_sql;
end;
/
 

SELECT FUN FROM DUAL
Output: 
least(OFFICE_PRODUCTS, OFFICE_ELECTRONICS)