Oracle's virtual machine service providing scalable compute resources. Like renting a powerful computer in Oracle's data center that you can configure for any workload.
Enterprises use OCI Compute Instances to run Oracle databases and enterprise applications in the cloud.
All are IaaS virtual machine services: you choose CPU/RAM, storage, and networking, then run operating systems and applications with full control. Differences are mainly in instance shapes, pricing models, and ecosystem integrations.