Configuring Theme for GTK applications on Archlinux with Plasma 5

There are two packages available to configure gtk applications to have oxygen theme: kde-gtk-config and kde-gtk-config-frameworks from aur.
You will also want to install oxygen-gtk2 and oxygen-gtk3 themes available in the arch repositories.
To configure the themes for gtk applications run the following command for kde-gtk-config:

kcmshell4 kde-gtk-config

For kde-gtk-config-frameworks

kcmshell5 kde-gtk-config

Basically kde-gtk-config-frameworks is built for kf5 and kde-gtk-config is for kde4. I use kde-gtk-config on plasma5 since it is in the arch repositories.
Note: to configure the theme of root gtk applications run the previous commands with kdesu.
Another thing to note is that the configuration files don't get created automatically so even if kde-gtk-config shows that it's configured gtk applications to use oxygen-gtk theme it might not be. Therefore change the themes for gtk2 and gtk3 applications to something else and apply then change it back oxygen-gtk if you wish.