You need to convert the string to date using the to_date() function
SELECT * FROM OrderArchive
WHERE OrderDate <= to_date('31-Dec-2014','DD-MON-YYYY');
OR
SELECT * FROM OrderArchive
WHERE OrderDate <= to_date('31 Dec 2014','DD MON YYYY');
OR
SELECT * FROM OrderArchive
WHERE OrderDate <= to_date('2014-12-31','yyyy-MM-dd');
This will work only if OrderDate is stored in Date format. If it is Varchar you should apply to_date() func on that column also like
SELECT * FROM OrderArchive
WHERE to_date(OrderDate,'yyyy-Mm-dd') <= to_date('2014-12-31','yyyy-MM-dd');