You can use the Database Gateway for ODBC (DG4ODBC) to cretae a database link from Oracle to MySQL.
Have a look at the following notes in My Oracle Support which give detalsl on the gateways in general and configuring DG4ODBC -
How to Setup DG4ODBC on Linux x86 32bit (Doc ID 466228.1)
How to Configure DG4ODBC on 64bit Unix OS (Linux, Solaris, AIX, HP-UX Itanium) to Connect to Non-Oracle Databases Post Install (Doc ID 561033.1)
How to Setup DG4ODBC (Oracle Database Gateway for ODBC) on Windows 32bit (Doc ID 466225.1)
How to Setup DG4ODBC (Oracle Database Gateway for ODBC) on 64bit Windows Operating Systems (Doc ID 1266572.1)
Detailed Overview of Connecting Oracle to MySQL Using DG4ODBC Database Link (Doc ID 1320645.1)
Please review these notes and get back if you have any further questions.