Hello. Currently kde-plasma/oxygen provides only Qt5 style. Upstream however still supports and updates Qt4 style as well. I am aware of kde-base/liboxygenstyle and kde-base/kstyles, which together provide Qt4 style, but they are quite old now. It would be nice to have an up-to-date version of oxygen Qt4 style. This will bring kde-plasma/oxygen on par with kde-plasma/breeze, which already provides Qt4 style via qt4 USE. Please fix.
Recently added kde-plasma/oxygen-5.7.3 brings USE=qt4, thanks for the report.