Following on from my previous posts, I've successfully set up a Raspberry Pi as a OpenWRT client device and also installed OpenVSwitch on top of it, with it all able to connect to the internet and download packages.
However, the installation process for OpenWRT also installed over the Raspberry Pi OS, meaning that it's effectively only usable as a networking device.
I want to stick an operating system and GUI back on that device, such that I can use it as a normal PC (or at least a normal Raspberry Pi) and run non-networking programs on there, whilst still being able to use it as an OpenWRT networking device.
Is that possible, and if so how would I go about it?
No. You can't.
OpenWRT is designed to works for embedded network devices with low resources. This mean that there was make some choices making impossible to run full GNU/Linux distribution on top of it.
Install a general-purpose Linux distribution.
Run OpenWrt as a VM or container if you still need it.
You would have to go for the virtualization path, as @vgaetera mentioned.
However, is it really worth it? Ideally you would want your router to be a dedicated device. You wouldn't like if your virtual router crashes or freezes because of an issue on the host OS or so.
Yes, most of cloud services nowadays run in virtualizations, but those are enterprise-grade system with dedicated engineers and redundancy in place, plus the cost factor considerd the large scale.
Yeah, well it's not 100% clear if you want to use both OS and OpenWRT at the same time.
If not there are many ways to multi boot openwrt (examples, more use search engine on these words)
Else yeah, VM or prob. preferred container could be used, but I agree with Hegabo that it's not something you rly want to use.