The code-page setting of the DB client must comply with the settings made in the database.
You can choose from either of the following three code pages, whichever best fits your needs:
WE8ISO8859P1, WE8ISO8859P15 and WE8MSWIN1252.
The choice depends on the characters you need to store in the database.
As WE8ISO8859P1 doesn't support the euro sign (€), WE8ISO8859P15 seems the better choice.
WE8MSWIN1252 supports not only the euro sign, but additional characters as well. So if you set your database up from scratch, this would be the recommended code page.
Please also refer to ORACLE's own support document no. 264294.1 dealing with the choice of code pages.
If you have your database already set up using WE8ISO8859P1 and you don't need the additional characters, there is no need to convert the database to a new character set.