Q:

How can I change the OCI Runtime in Podman

I know that in Docker if you want to change the runtime you go to: /etc/docker/daemon.json
And modify: 
{
    "runtimes": {
        "runnc": {
            "path": "/usr/sbin/runnc"
        }
    }
}

But how do I do this with Podman (or just add runtime)?

OCIRuntime: package:
runc: /usr/sbin/runc
path: /usr/sbin/runc

A:

You can find the runtime_path defaults in /usr/share/containers/libpod.conf and overwrite them in /etc/containers/libpod.conf (on EL8, check man podman to find correct place for Your setup).
As there is no daemon this is the conifguration used for all the libpod commands that need to know that.

Edit: as stated by @jnbdz the probably more important default location: in rootless mode $HOME/.config/containers/libpod.conf

