With Plasma 5.3.2 under Manjaro Linux, I have discovered two problems about window titles when using the Oxygen decorations.

The first one: the expected behaviour is that the title becomes darker when a particular window is inactive (out of focus) and it turns brighter when it is active again. This seems to be broken. The only cases when the title changes colour are when I resize the window or the text of the title is updated by the application (for instance, there is a media player playing music, whose window is inactive, and it changes tracks in a playlist).

The second one is that the length of the text of the window titles are limited. For example, I have "Plasma 5 · KDE Community Forums - Mozilla Fir…" written in the titlebar when browsing this subforum.

I have none of these problems when Breeze is set as the window decoration theme.

You may want to file issues on bugs.kde.org to let the developers know about them.