I'm using Transparent Oxygen, and almost everything is transparent, but there are a few parts that aren't, like Dolphins file/folder area, the text area of quassel, etc...

So, those areas are using the color of my theme instead of being transparent. I tried to modify my current theme, but I can't change the color translucency.It's like there was no RGBA support.

Is this possible?

Using 4.6 vainilla on Arch

Maybe I should change my question. Does KDE Colour Scheme support RGBA? Or even better: Can I make a translucent colour scheme and import it from KDE's Style & Aappearence config ?

generally only those parts of the window that use the Window color from the color scheme (the back of the window itself - upon which other widgets are drawn) will be transparent. Any buttons or views (e.g. text areas, the files pane in dolphin or konqueror, the html view area in konqueror, etc.) will not be transparent. There currently isn't support for RGBA for other parts of the window (the alpha part of the window background isn't controlled by the color scheme either, but by the style engine itself)

Is the style engine (in my case) Oxygen Transparent? Or it's something different? If yes, is there any posibility to patch it and make it support RGBA on those areas too?
If no, is there any way that I could make those areas translucent too?

Oxygen Transparent is a style engine. I suppose there may be the possibility of allowing RGBA for other widgets - post a request in the Brainstorm forum.
On the other hand - you can use that across any style engine as well as on gtk or other toolkit apps as it is handled by the compositor of the window manager.

You should probably goto systemsettings->desktop-effects->All Effects->Translucency. Click on that spanner ("wrench", if you love google-chrome). Set your transparency.

That's true - just use the desktop effects to set the transparency for the whole window; though the trouble there is that everything, including text will also be transparent - rendering a 100% transparent window 100% unusable.

No, there is a difference between transparent and translucent, and there is a difference between the transparency plugin on KDE and Oxygen-Transparent. (Try it and you'll see).

Right, there are differences. I was trying to suggest a quick-fix