The procedure is to login to User Management using the default password you will find in the documentation and from there create the account for OAM Admin.
Note that you cannot access User management and OAM Admin pages with the same username. The pages may seem the same but the username you type when you login defines which of the two (User Managment or OAM) you will access...
The procedure is described in chapter 3 of the Admin guide for AES 3.1 (not 3.0)
You can also do the following:
Login to linux CLI, go sroot or root, depending on the bandle you have, and create a new usergroup and new username/password (for example oamadmin) and try the new account for OAM..
Commands are (for group oamgroup, user oamadmin)
groopadd oamgroup (adds group oamgroup)
useradd -g oamgroup oamadmin (adds to group oamgroup the user oamadmin)
passwd oamadmin (this will force you to set a password)
Petran.