Support for Oracle18c by Delphix virtualization and masking products is on our product roadmap.
As Oracle18c was made generally available only a week prior to when this question was asked in late February 2018, most vendors supporting Oracle software have yet to announce support for this newest version of Oracle Database.
Generally speaking, Delphix will supports newly-announced OS/RDBMS about 6 months after the general availability (GA) release.