Since we are talking about spelling, it is "hors d'œuvre" and not "hors d'oeuvre".
On macos: Option + q for œ
On macos: Option + q for œ
So. <compose>oe = œ
I usually bind capslock to the compose key in Linux (and Windows with Wincompose). Hacker's Keyboard has a dedicated button.
... and then, in my Firefox configuration, unexpectedly navigates me to HN's /show page.
FWIW, Option + O yields: Ø ... and then performs the same navigation.
Press Opt-o, then let go of opt. Then select the œ, which happens to be option 6.