Since a few days the Certification Matrix for Oracle Fusion Middleware 12c Release 2 (12.2.1.x) was updated within Oracle Technology Network, now Oracle 18c (18.1 on Exadata and 18.3 on On-Premise) is certified and supported as Target Database for RCU (Repository Creation Utility and as Application Datasource.
Certification Matrix for Fusion Middleware 184.108.40.206.0: https://www.oracle.com/technetwork/middleware/fusion-middleware/documentation/fmw-122120-certmatrix-3254735.xlsx. Read the complete article here.
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