Installing Oracle XE 11gR2 on Fedora 15 - linux group permission deadlock
You've got the distinction between users and groups mixed up. Putting the dba user in the oinstall group will not mean that the dba group will also have permission to write to /u01/app/oracle when the owning group is oinstall - this is because the dba user and the dba group are completely separate entities.

The Oracle user should be in the oinstall and dba groups. There shouldn't be a dba user, only an Oracle user.

chown oracle:dba /u01/app/oracle


... will fix it.