Hi,
when I try to do " su - oradba" on my solaris box i get following error:
Please let me know the proabbale cause and resolution to this.

su - oradba
Enter Local Unix Password:
Login Error
-----------
The user 'oradba' is not allowed to log direct into this host.
If this is in error, then please contact Unix Support, x7989.
This login attempt has been logged.

your solution is right there:

your administrator has configured the system this way...

Your admin will have to grant ( to you) the rights needed to do:

pfexec su - oradba

If s/he has not done so already.