I know it got delayed a bit. But there’s a good reason for the delay. In addition, my post is a bit late as well but I was on vacation. Anyway, you can download it since Aug 22, 2018. Oracle Database 18c on-prem for MS Windows is available.
Oracle Database 18c on-prem for MS Windows is available
You can download it from oracle.com and eDelivery:
Start here with the Oracle 18c on Windows Installation Guide.
As far as I see, the certification information has not been updated yet in MOS Note: 1307195.1 – Certification Information for Oracle Database on Microsoft Windows x64 (64-bit) but should be included soon.
You can find the certification information instead with the MOS Certification Tab. It displays these Windows versions as certified for Oracle Database 18c: