Setting passwordless ssh root connection using dcli is fast and simple and will easy later to execute commands on all servers using this utility.
In order to do that you should have either:
DNS resolution to all Database and Storage nodes OR have them registered in /etc/hosts
1) Create a parameter file that contains all the server names you want to reach via dcli, tipically we have a cell_group for storage cells, a dbs_group for database servers and an all_group for both of them.
The parameter files will have only the server name, in short format
ie: all_group will have on an Exadata quarter rack:
2) As root user create ssh equivalence:
ssh-keygen -t rsa
3) Distribute the key to all servers
dcli -g ./all_group -l root -k -s '-o StrictHostKeyChecking=no'
dcli -g all_group -l root hostname