kde-plasma/oxygen-fonts currently has DEPEND="media-gfx/fontforge". mreug says the ttf files are in the tarball, so why do you need fontforge?
It looks like CMakeLists needs some reworking so that it doesn't bail out when fontforge is missing. Also, there are some "fake" targets for generating the .ttf files that need to be adjusted to point at the right path and so that the rules will not fire when the .ttf files are already up to date.
We currently do it this way because of this upstream change: http://quickgit.kde.org/?p=oxygen-fonts.git&a=commitdiff&h=67fb96b5db911d91046c32233e55585845d6f8c3, and this is also consistent with other distros.
(In reply to Michael Palimaka (kensington) from comment #2) So why are the .ttf files included in the repository/tarball? It seems quite strange to include them if they will never be used.
I've spoken to upstream, and they've advised the bundled fonts are left over and recommend using fontforge.