I have setup Opnsense on my Virtualbox with two interfaces: NAT and Host-only. I have configured my two interfaces via Opnsense console as NAT being WAN and Host-only being LAN. I am able to ping 8.8.8.8, however I cannot open the WebUI using the LAN IP from the host computer.
I have solved the problem. For anyone that may face the same problem, here is the solution:
I have changed the IP of my host-only netowrk from Virtualbox settings and set my WAN address in Opnsense accordingly