kde plasma workspace - Why are my monospace fonts behaving erratically?

I'm setting up an Arch box and I've installed Plasma 5 as my desktop environment. However, monospace fonts are behaving very erratically:

• Listed in the 'Fonts' prefpane, it displays fine.


• As part of a terminal emulator, it's fine.


• Firefox slants them something awful in some cases (see forum), and


• will completely miss them in other cases (see post).

It's probably worthwhile to note that evaluating (set-default-font "Oxygen Mono") in emacs fails with

Invalid font name: -unknown-Oxygen-Sans-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1,

suggesting to me that there is some font registration issue. (Note that it's looking for Oxygen Sans…)

How can I achieve sanity and convince these fonts to behave?

Additional information from continuing research:

• $ fc-match mono gives Oxygen-Sans.tff: "Oxygen Sans" "Sans-Book"


• $ fc-match foo also gives Oxygen-Sans.tff: "Oxygen Sans" "Sans-Book"