Effective January18 2019, Oracle changed the way in which it distributes the configuration information for yum repositories provided by the Oracle Linux yum server. This document explains what you need to do:
https://docs.oracle.com/cd/E52668_01/F14052/html/notice_description.html
See also: https://blogs.oracle.com/linux/modularizing-the-oracle-linux-yum-server-repository-configurations-breaking-up-is-hard-to…