As we already know that 18c for Cloud and On-Premises Engineered System was released earlier this year in February/March.
18c for On-Premises Engineered System is available for download on edelivery.oracle.com
Now Oracle database 18c for on-premises (Linux x86-64) is available for download on OTN.
18c is nothing but 126.96.36.199. Instead of releasing 188.8.131.52 Oracle implemented new numbering system for future database releases beginning of 2018. New format will consist of Year.Update.Revision e.g. 18.1.0
Since this version is being released in Q3 of 2018, we can see that it is called 18.3.
You can find all 18c new features in the documentation.