We don't officially support IPv6, but a lot of the stuff OCI does will work across it.
We basically made a large IPv6 audit a few years ago, but a variety of 3rd party components were not IP v6 ready at that time. We have not gone back to finish the audit as there has been a real lack of customer demand.
One thing that does not work great with IPv6 is auto host resolution - we do not lookup AAAA IPv6 address records if DNS validation is enabled.
Overlapping IP address ranges - the answer is "maybe". Depending on how your environment is, that may or may not be a problem.
What types of devices are likely to have their IP addresses overlap?