Once you create  Oracle Cloud service instances,

To access your instances using SSH, generate a SSH key pair, associate the public key with your instances, and use the private key to log in to the instances using SSH.

https://docs.oracle.com/cloud/latest/stcomputecs/STCSG/GUID-EE29085A-79B1-4A3A-BF25-A2A9516EC5F3.htm#OCSUG149

Generating an SSH Key Pair on UNIX and UNIX-Like Systems

Generating an SSH Key Pair on Windows