What file contains the configuration for the network interface em0on an OpenBSD system'?

The file /etc/hostname.em0 contains the configuration for the network interface em0 on an OpenBSD system. This file specifies the IP address, netmask, gateway, and other options for the interface. The file is read by the netstart script during boot or when the interface is restarted. The file name is derived from the interface name, which is usually based on the driver name and the device number. For example, em0 is the first interface using the em driver, which supports Intel PRO/1000 network adapters.

References:

Networking | OpenBSD Handbook

[hostname.if(5) - OpenBSD manual pages]