In-place upgrading can be a bit tricky with Oracle Linux, but should in theory work from CentOS7 to Oracle Linux 8.
You can take a look at Upgrading and Updating Foreman for a general guide on how to do an in-place upgrade and Possible to use Leapp for Oracle 7 -> Oracle 8 migration? for some additional hints on possible Oracle Linux pitfalls. It is a lengthy thread, but most of the content way derailed to handling a missing steps from the upgrade documentation that has since been added.
If you try to go that route, I highly recommend taking a snapshot or full machine backup beforehand, so you have a way back in case something goes wrong.
