Hi,short answer: Yes, it is the IP of the resolver. 127.0.0.1 is the 'local host' which is your OPNsense.BrReza(Interfaces -> Diagnostics -> DNS Lookup? ...)
if you enable dhcp service on OPN, the lease contains the dns server for the client receiving the lease to use.Some don't respect that, might have an address hardcoded. For those, the rule is a mechanism to "catch" the request and redirect it to where you want it.Please note you haven't specified where you are doing the lookup. Client or OPN could have a different explanation.