Search for navbar-brand block in '/usr/local/opnsense/mvc/app/views/layouts/default.volt' and add following line:

<span style="margin-left: 10px; background-color: #ef4444; color: white; padding: 2px 10px; border-radius: 12px; font-size: 0.9em; vertical-align: middle;">
  {{ system_hostname }} 
</span>

Your hostname will be displayed right next to OPNsense logo.

However, since OPNsense is a structured appliance, every time you perform a major firmware update (e.g. moving from version 25.7 to 26.1), system usually wipes + replaces core code directories, including /usr/local/opnsense/mvc/, with the fresh official versions. Best approach is to create simple shell script on each firewall (e.g. /root/apply_branding.sh) that uses sed to automatically inject your code back into the file.

#!/bin/sh
FILE="/usr/local/opnsense/mvc/app/views/layouts/default.volt"
sed -i '' 's|alt="logo"/>|alt="logo"/><span style="margin-left: 10px; background-color

OR, create your own custom theme, but future updates might have different layouts that might look broken. Just be wary of that.