Try to put your current query in subquery as follows :
SELECT * FROM (
TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') AS HISTORY_DATE
FROM HISTORY WHERE
STORAGE_GB IS NOT NULL AND
APP_ID NOT IN (SELECT APP_ID FROM HISTORY WHERE TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') ='06.02.2009')
ORDER BY STORAGE_GB DESC )
WHERE ROWNUM <= 10
In Oracle, rownum is used to the result after it is returned. You just have to filter the result after it has been returned, therefore the subquery will be required.
You could also use the RANK() function to get Top-N results. For performance, you can use NOT EXISTS in place of NOT IN.