Your screenshot does look ugly indeed. Not at all how my LibreOffice looks in KDE4 on slackware-current here in the home.
Two libraries in the libreoffice package take care of KDE4 UI integration:
These are only compiled when you build LibreOfffice on a computer with KDE4 installed. In my package, those libraries are present.
Oxygen-GTK does not influence the UI of LibreOFfice in KDE.