Oxygen (as opposed to, say, Plastique) tooltips has no borders and this cannot be configured. I think there should be an option to specify tooltip border color.
KDE: 4.8.4 (4.8.4)
Steps to Reproduce:
1. Set Oxygen widget style.
2. Turn off KWin compositing.
3. Hover any item in a GUI that has a tooltip info.
Tooltip appears without any border which looks pretty weird when tooltip's and window's background color is the same.
Tooltip should have a border.
Tooltips use a shadow, so on very dark color themes, it should probably use a glow instead (as window decorations).
Glow is for active windows, which tooltips are not. Besides the bug report is about compositing turned off, so no shadow/glow.
My suggestion would be to just change the color scheme to use high contrast background for tooltips wrt window, since there are dedicated settings for that in the color scheme.
As far as I know there is no solution within oxygen, that would work with all colorscheme any way.
So closing as wontfix, sorry.
PS actually, tooltips do have a 1 pixel border (contrast), that is only visible however, if the background color is not too dark.
So, there is no solution right now so you won't do and change it and just close this bug instead? The hell reasoning is that? This is seriously dumb. Whatever, suit yourself.