It is difficult to give useful advice for an interview
where the interviewer may not have the technical background to understand the answers.

These suggestions are provided in the absence of anything better :-
The answer comes in a number of parts :-
1) Check out this book :- 
   'Ace the Technical Interview - Database Edition',
   published in March, 2000 (ISBN: 0071352406) gives you the story from 
   the interviewee's perspective but will therefore provide some useful insights.
2) Here's another book :- 

Conducting the Oracle Job Interview: IT Manager's Guide, by Mike Ault of TUSC, 
and Don Burleson, published in August,2003.
3) Check out this Page for an idea of the sort of personal characteristics you are looking for :-
      http://www.databaseanswers.com/what_is_a_dba.htm
    You are looking for someone whose orientation is towards technical matters rather
    than people. The DBA often has to play a regulatory or 'police' type role and it's 
    sometimes difficult to be a popular team member under those conditions.
4) Ask 'What is a typical day in your job, and which parts do you like the most and the least ?'
   Check this page for some possible answers :-
       http://www.databaseanswers.com/a_day_in_the_life.htm
5) Ask 'How would you identify the bottlenecks in Database performance ?'
   The answer is to examine the Oracle Execution Plan, and to make sure that the 
   appropriate Indexes exist and are used by the Oracle Optimizer.
6) Ask 'How do you find the numbert of rows in a Table ?'
   A bad answer is count them (SELECT COUNT(*) FROM table_name)
   A good answer is :-
   'By generating SQL to ANALYZE TABLE table_name COUNT STATISTICS
    by querying Oracle System Catalogues (e.g. USER_TABLES or ALL_TABLES).
   The best answer is to refer to the utility which Oracle released which makes it
   unnecessary to do ANALYZE TABLE for each Table individually.