You can't used database specific commands (e.g., TO_CHAR) in a SQL statement using an ODBC driver What you have to do is specify the ODBC equivalent than then let the ODBC driver convert it for you. For example, if you do:
WHERE {fn CONVERT(EMPNO,SQL_CHAR)}
Then what the ODBC driver should pass when it's talking to Oracle is:
WHERE to_char(EMPNO)