Code:
SELECT DISTINCT a.QUARTER_NUM 
FROM a,b 
WHERE a.PERIOD_YEAR='2001' 
AND SUBSTR(b.ATTRIBUTE9(+),5,1)=a.QUARTER_NUM 
AND b.CODE_COMBINATION_ID(+)=4356
Oracle documentation:
If the WHERE clause contains a condition that compares a column from table B with a constant, the (+) operator must be applied to the column so that Oracle returns the rows from table A for which it has generated NULLs for this column. Otherwise Oracle will return only the results of a simple join.