If you are unshure as into which rdbms you want to restore the data an you do not want to dump the structure, you could dump it into a file on the oracle server using an external table (type oracle_datapump) as a .csv-file. That should be easy to import into any rdbms.
Another way would be to store data as (ANSI-)sql import statements (which, like xml, are rather verbose, but compress well). Those statements can be edited easily afterwards.
You could dump the ddl for the structure into a text file using either exp/imp or dbms_metadata.get_ddl('table', 'schema', 'user'), but you would have to do a lot of adjustment on the (Oracle-) ddl to adapt to other rdbms.