Oracle database link to MySQL database on RedHat Linux
Oracle can connect to MySql with an ODBC connector.  This is described in some detail here.  Also see the official Oracle documentation here.

This will work for Oracle 10 and 11g to MySQL 4.1, 5.0, 5.1 or 6.0.  There are limitations both in performance and what you can do.  My understanding is


no distributed transactions
no insert as select .....
treat numbers as numbers and strings as strings, this is not the place for implicit type casts
as you might expect non-Oracle features are not supported
no Oracle DDL is supported
these functions are supported AVG(exp), LIKE(exp), COUNT(*), MAX(exp), MIN(exp), NOT