Glad you liked the look! I actually found that specific crescent moon icon (\udb82\udd79) by searching the Nerd Fonts Cheat Sheet. I thought it captured the spirit of the CachyOS logo well enough to use as a placeholder while we wait for official support from the Nerd Fonts team.
I don’t actually use the Oh My Zsh + Powerlevel10k combo. I switched to Starship, which makes customizing these things very straightforward. In Starship, you can simply define the symbol in your starship.toml like this:
[os.symbols]
CachyOS = " <icon> "
Since I’m not on P10k currently, I’m not 100% sure of the exact variable name, but you can usually override the OS icon by setting POWERLEVEL9K_OS_ICON_CONTENT='' in your .p10k.zsh file. You might need to look for the section that handles os_icon and manually inject the glyph there.
Hope this helps you get your setup looking right!