Please add an ebuild kde-base/katepart-4.14.3-r1 to kde overlay with the above dependencies, otherwise katepart tries to pull in kde-base/oxygen-icons-4.14.3 which is blocked by kde-apps/oxygen-icons.
Alternatively, I guess you could already import the overlay's kde4-base eclass to the main portage tree (although I don't know how that would play out for installations without kde overlay present).
Please paste your error message - this is already handled in the eclass (in the main tree): if [[ ${PN} != oxygen-icons ]]; then kderdepend+=" || ( kde-apps/oxygen-icons $(add_kdebase_dep oxygen-icons) )" fi
Huh, now it's gone. Recompiling katepart probably resolved it. Please close, sorry 'bout that.