Fault domains can protect your application against unexpected hardware failures or outages caused by maintenance on the underlying computer hardware. Additionally, you can launch instances of all shapes within a fault domain. Oracle Cloud Infrastructure is typically designed with three availability domains per region, and each availability domain has three fault domains. When carrying out maintenance on the underlying computer hardware, Oracle Cloud Infrastructure ensures that only a single fault domain is impacted at one time to guarantee availability of your instances in the remaining fault domains.

Getting started is easy. When you create a new compute instance using the API, CLI or Console, you can specify the fault domain in which to place the instance. If you don't specify a fault domain, the instance will be distributed automatically in one of the three fault domains within that availability domain. Oracle Cloud Infrastructure makes a best-effort anti-affinity placement while optimizing for available capacity in the Availability Domain.
Reference:
https://blogs.oracle.com/cloud-infrastructure/introducing-fault-domains-for-virtual-machine-and-bare- metal-instances