Get to know your database structures. Number of instances that are currently running on your site. What are the depending applications and how cretical their functionality to your organization. Know the tools they us to monitor the database and get some aquaitence with them. Always keep a note of what you do? If u have to delete/update/drop any record/table/schema, always double confirm it and make a temp copy of it such that you would not land into problems in case of mistakes. Then the location of your backup scripts and the type of backup they do. Always be ready to involve in the disaster recovery process. Talk to your DBA regaring the methodologies, and the company practice. Get to practice the recovery process in your test database atleast once a month. Talk to your dba, find out how they get the system alerts, and if there are some mailing list, make a request to add you in that. Check the alert logs of your instance for any possible problems. Know your oracle support CSI/username and password, so that you can go and create tar in case of emmergencies

I know it is too much to know, don't worry, if it is going to take some time.

Congratulations! and good luck.

Sam