For the whole Exadata rack serial number:

ipmitool sunoem cli 'show /SP system_identifier'

For the server serial number you are connected to:

ipmitool sunoem cli "show /SYS product_serial_number"

For each component serial number on a single server:

You can also get the individual serial numbers for each component in an Exadata Rack like this:

/opt/oracle.SupportTools/CheckHWnFWProfile -S

Note: Run those commands with root user