I assume that Oauth Calls most likely means OAUTH in the context of V$TOPLEVELCALL, rather than Open Authorization https://en.wikipedia.org/wiki/OAuth in this instance. Given the wait events and symptoms, then the most likely explanation is a connection storm, with the high level of kernel activity being due to a high frequency of system calls when creating new foreground processes which then need to map the SGA and so on. A high level of kernel activity during a connection storm isn't specific to Solaris or Oracle Database 19c. In most cases, the best recommendation is to find and fix whatever triggered the storm, rather than focusing on what happened when the storm was well-established.

Thanks to Mike Hallas for his assistance here.