If I recall correctly, Oracle licencing would actually require to pay for all the 288 cores on the host even if just a subset of the VMs in that virtualization run Oracle.
True for all x86 virtualization i know of.
But not true for IBM Power LPARs where only the entitled CPU resources have to be licensed and can even be shared between LPAR (Power Enterprise Pool).
I don't know for sure but would therefore assume that it's the same for the IBM mainframe platform.