SQL & PL/SQL

If your MySQL database is accessible through an ODBC-driver, then you could configure DG4ODBC, to provide you with transparent SQL access to that database using a regular database link.

However, I don't think that you'll be able to perform a distributed transaction (involving 2 phase-commit). So you'll have to commit the work in Oracle first, and then do a remote transaction via the db-link, and commit that too.

http://download.oracle.com/docs/cd/B28359_01/gateways.111/b31042/configodbc.htm

We use UnixODBC and Oracle's Heterogeneous Database Services to create a database link from inside an Oracle database to a remote mySQL database.

Simple explanation of how it works:
A database session uses a database link to mySQL. The database link refers to a HS (Heterogeneous Service) supported by the local Oracle Listener. The Listener hands that connection off to a HS Agent. This Agent handles the database connection request, opens an ODBC connection to the foreign database server, execute the SQL (via ODBC), fetches the response and then returns it to the database session.

So what you need to do are:
- configure ODBC connectivity from your Oracle server to the mySQL server (create a DSN and test it using an ODBC client like isql)
- install and configure the HS Agent (which needs to use the ODBC libraries and the DSN)
- configure the Oracle Listener for supporting the HS Agent
- define the database link and test it

If you google this topic, you should find the relevant references to Oracle documentation and even blog entries with examples.