In order to give static IPs to devices with hostnames, you have 2 options
Option 1
If used static IPs in devices itself, you can override them to put custom hosts.
To do that, go here in OPNSense
Services → Unbound DNS → Overrides → Host Overrides
You also have Alias option, which is used for having multiple domain names to a single IP. You can define a host override to add a domain to IP, and create Alias override to have multiple domains.
Option 2
Reserve specific IP addresses to MAC addresses, so even devices that want to use static IP can enable DHCP.
Services → DHCPv4 → [Your Interface]
In DHCP Static Mappings, add the host.