Being fairly new to home networking, this took me hours to crack... but no doubt common-sense to most of you on here :)
Requirement: iPhone OpenVPN client to use local OPNsense DNS arrangements (Adguard etc).
Solution: set "DNS Servers" under OpenVPN Servers to "10.10.0.1" - or whatever the IP address is of the local OpenVPN server.
No other changes needed... I thought it would just work with 192.168.1.1 - and tried everything such as push-option DNS etc...
Also explains why I was getting "reply from unexpected source" response when using nslookup from my iPhone (when forcing 192.168.1.1 DNS).
This helped me to get local DNS and outbound DNS working. Thanks :)