What is needed to port this to Kf5?

I'm very interested to see this happening as well! Subscribed to the thread, in case there might be updates here.

Checking the repository (https://projects.kde.org/projects/playg ... repository), for activity, the last (substantial) changes were a little over a year ago.
Similarly on the kde-look page (http://kde-look.org/content/show.php?content=127752), the author hasn't responded to the last few requests for updates (apparently it needs some changes to work properly with KDE 4.13 and later as well).

In short, it appears that Hugo has abandoned work on oxygen-transparent (for work on VDG).

Do you need a theme with transparency support which specifically uses the oxygen-styled widgets or would it be sufficient to have a theme that works with Qt/KF 5 applications?
There's Virtuality (http://kde-look.org/content/show.php/Vi ... ent=165086), which is inspired by bespin + KF5/VDG, supporting transparency and both Qt4 and Qt5. It's a bit of an acquired taste though.
You might also ask in the VDG forum about the possibility of adding transparency support to the Breeze theme.

If it must be Oxygen-styled widgets, you might try contacting Hugo directly about updating Oxygen-transparent to work in Qt5.

To be honest, I'm personally more interested in seeing a breeze-transparent. But I assume that's even less likely to happen

I would imagine that breeze-transparent is more likely to happen than oxygen-transparent since breeze is the current hotness.

Of course, my personal preference would be Bespin for Qt5, but the author has mentioned in such a feature request that the Qt5 successor to Bespin is Virtuality. Perhaps I might just take that that as a personal side project after I've upgraded to KF5 (waiting for stable, complete release made available for kubuntu 14.04, which I wouldn't expect until after the summer release, or later).