The 18c Oracle DBaaS is now available. This is the only place that Oracle 18c will be available until later in 2018. So if you want to try it out, then you are going to need to get some Oracle Cloud credits, or you may already have a paying account for Oracle Cloud.
This is a companion discussion topic for the original entry at https://blog.toadworld.com/2018/03/02/oracle-18c-dbaas-cloud-setup