Yes a oracle DBA can manage an app database provided it is a small organisation but what will happen if the orgination is having number of databases and applications.Then the role of Application DBA will come into picture and will cooradinate between a DBA and application design team.
Still there are lot of things are there which can't be expalin in one paragraph.
search for this topic as it is already discussed on this forum.
Also check on the following link.A small discussion on this topic will clear some of ur doubts on this. http://www.mail-archive.com/oracle-l.../msg45841.html
What is the difference between normal Oracle DBA & Oracle Apps DBA?
Can't a normal DBA manage Oracle Apps database?
From a PeopleSh*t persepctive I can tell you that I wenr from being Joe DBA to Joe WebLogic Admin, Joe Application Server Admin, yadda yadda yadda as these components are rarely segregated in their adminisration.