It is very demanding to be a database administrator, or DBA.
You first need to be an expert on the database software itself, in this case the database is Oracle.
The primary mission of a database is to store information on a disk, and retrieve it when needed. So you will soon find that you need to learn about disk storage, including such things as

1. RAID,
2. disk mirroring, and
3. striping.

The efficient use of memory is critical to Oracle's performance, giving you another subject area to learn about. You will need to back up your database, which might lead you into the world of tape drives and backup software. Users will want to connect to your database. Usually that's done over a network—one more thing to learn about, and so it goes. To be a DBA, you have to constantly be in a learning mode.