OpenMediaVault default password is printed in the documentation. I did not see it the first time I installed it either. So I had to do some detective work my self to find it.

OpenMediaVault default password

For the Admin panel
Username: admin
Password: openmediavault

SSH/Root
Username: root
Password: (You selected your own root password during installation)

Some useful links

OpenMediaVault documentation.

How to change the admin password.

How to set a new root password if you have lost your root password. Only for Debian based installations.