Raveendra,
this will require an extra step. Please download "MySQL GUI Tools Bundle for 5.0" from here,
http://dev.mysql.com/downloads/gui-tools/5.0.html and install it.
Then run the MySQL Migration Toolkit that is part of this bundle. It will allow you to connect to your Oracle server once you have installed the Oracle JDBC driver (see it's manual). Once connected, you can select the schemata to be reverse engineered and converted to the MySQL SQL dialect. Let this be written to an SQL file on disk.
This SQL file can then be reverse engineered within MySQL Workbench with a single step.
Direct support for Oracle is planned, but not a top priority at this point in time. If you have acquired a WB license just to use the tool with Oracle, you may want to ask for a refund and get a tool with full Oracle support like
http://www.sybase.com/products/modelingdevelopment/powerdesigner instead. But if you plan to work with MySQL in the future anyway, I hope you will keep your license.
Hope this helps,
Mike
Michael Zinner, Team Lead, Developer Tools
MySQL AB, www.mysql.com
Are you MySQL certified? www.mysql.com/certification