snmpwalk -Os -c public -v 1 x.x.x.x ssCpuSystem.0
close
since it ends in a '.0' they call it a 'scalar' and you should fetch it as either:
snmpget ... ssCpuSystem.0
OR
snmpgetnext ... ssCpuSystem
(ie, you use the .0 form for GET but you 'go back one' in the OID tree if you are using get-next style walking).