There are some things a good DBA must know.
1. This task? its easy, go to Google, DB Forums, etc.
But I know, however, that it cannot be a replacement for foundational knowledge of your chosen database server.
Sure, I can (and sometimes do) google the syntax for a command to get it right, but foundational knowledge is too important to be left to searches.
2. Continuing education is very Crucial, be aware of your technology.
This is in continue of point #1. Things change, even in the world of Oracle, so keep yourself up to date on the latest releases, patches, etc.
3. Monitoring is critical.It doesn't matter which monitoring tool/system you use(OEM, Grid Control, third party tools, etc), just make sure you have one in place and it should be working! You should know when a server has too many connections, not enough memory, or if it's running out of disk space (or a hundred other things). Montoring is the base of tuning.
4. There is nothing more important than backups.
Performance tuning and building an awesome server are cool, but this is the bottom line: our data is sacred. Before anything else, you have to make regular backups and you have to make certain that those backups work properly. You must have a backup strategy, verify all the scheduled backups run successfully(by checking the backup logs) on a daily basis and perform test recoveries if needed to check the backup status.
5. Have fun at your job.
I hope you do what you do because it's your passion. It is for me, when I manage databases I am protecting client's most valuable resource and always learning something new. I get satisfaction from what I do. Because it matters a lot, whether you like your work or not.