oracle linux public yum repo info has been modularised

since jan 2019 oracle linux public yum repo info has been modularised

https://docs.oracle.com/cd/E52668_01/F14052/html/notice_description.html

You should update your systems to use the modular yum configuration as soon as possible.

new oracle linux yum docs

and if installing on virtualbox from the oracle linux 7 ISO you should enable networking in your vm and use the following oracle linux public yum network location

when virtualbox guest additions says it needs kernel headers what it really means is that it needs kernel- devel

# yum install kernel-devel