The one with 4g built in is currently not supported (esim) but the bluetooth one is independant of any telco.
If/when Optus releases the cellular version it should show up here with the Apple ones:
The Optus parent company(singtel) has a monopoly agreement with Apple to not allow any other brand. They shut off Samsung watches overseas that were connected at that time. There is little to no chance of it changing and allowing connection.