I can't get a clear answer on this one...

I have a Oracle user created in group 'dba'

when this user touches a file the group displayed is 'sys' - why?

The 'sys' group is not included in the list of secondary groups for this user.

Is this standard to Oracle on Unix? (AIX)

Anybody?

You have a very serious problem. In your /etc/group file you have two lines, one for "sys" and one for "dba" that both have the same group id. When you added the dba group to your system you were supposed to insure that it had a unique group id. When you have something like:
sys::3:
dba::3:
then the dba group is really just an extention of the sys group.

You need to assign a unique group id to the dba group. Then you need to run chgrp on all those oracle files to get set to the right group.

Your /dev/mem and /dev/kmem files may be readable by the group sys; if so, this is a real security hole.

But, on the bright side, if you ever have a group with too many users to fit on one line in /etc/group, you will know what to do