If you've created your instance using an Oracle-provided Oracle Linux image, then you can log in to your instance using SSH as the opc user.
https://docs.oracle.com/en/cloud/iaas-classic/compute-iaas-cloud/stcsg/accessing-oracle-linux-instance-using-ssh.html#GUID-D947E2CC-0D4C-43F4-B2A9-A517037D6C11
https://docs.cloud.oracle.com/en-us/iaas/Content/Compute/References/bestpracticescompute.htm