IsOneWay should be enough, but it still can block your client in specific circumstances, as well as throw errors.
See this and this posts for more details:
General things you should keep in mind about OneWay operations -
O/W operations must return void. O/W operations can still yield exceptions. invoking an operation on the client channel might still throw an exception if it couldn’t transmit the call over to the service. O/W operations can still block. if the service is pumped with messages and a queue had started, calling O/W operation may block your following code.