Opnsense is Hardened FreeBSD, so there is no choice. If it was me I'd run both as VMs, one VM for Opnsense and I would use Ubuntu to host the webserver on a second VM. One Virtual switch connected to one of your NICs as the LAN side and another for the WAN side for Opnsense. There are various Hypervisors around, I've used ESXI on baremetal in the past to do what you want to do, not the most friendly to set up but it works. VirtualBox is fine, but if you are running Windows10 Pro you can use HyperV which is pretty easy to set up - I use it for testing some scenarios.