What is the default ONIE username password for S5448 S5232 S5248 switches ?

The default username of ONIE is root. There is no password that is required.

In some ONIE releases and platforms like S5400, Z9400, Z9600 the ONIE default username is root and password is ! (all letters uppercase)

For example: Consider service tag of switch is X123YYX, then ONIE password is X123YYX!

https://www.dell.com/support/kbdoc/en-hn/000193191/dell-emc-networking-on-switches-default-username-of-onie-for-remote-management

Hello, to access the switch through a serial console, there is no required password, but to access the switch remotely need username/password as link shared by Fire_Born.