A region is a localized geographic area, and an availability domain is one or more data centers located within a region.
Because availability domains do not share infrastructure such as power or cooling, or the internal availability domain network, a failure at one availability domain within a region is unlikely to impact the availability of the others within the same region.
Oracle Cloud Infrastructure is hosted in regions and availability domains. A region is a localized geographic area. And an availability domain is one or more data centers located within a region. Our region is composed of one or more available domains. Most cloud infrastructure resources are either region-specific, such as virtual cloud networks, or availability-domain-specific, such as compute instance.
Traffic between availability domains and between regions is encrypted. Availability domains are isolated from each other, fault tolerant, and very unlikely to fail simultaneously. Because availability domains do not share infrastructure such as power or cooling, or the internal availability domain network, a failure at one domain within a region is unlikely to impact the availability of the others within the same region.
https://docs.cloud.oracle.com/iaas/Content/General/Concepts/regions.htm