Yes, something like it currently exists, but it is not the same resize grip in the corner that is used in gtk themes. Something like that would need to be coded into openbox.
Add a Handle
In your themerc for the theme you are using, you can set
window.handle.width to a number of pixels and handle will appear below the window1. The handle includes diagonal resizing tools in the left and right corners. Unfortunately, this method does take up a bit more real estate than the gtk style corner grip.
For example, in your themerc, this would create a 6 pixel wide handle:
Specifies the size of the window handle. The window handle is the
piece of decorations on the bottom of windows. A value of 0 means that
no handle is shown.
To activate changes made to the theme, run
Change Border Width
You may also change the
border.width setting in your themerc to make the window border wider. This increases the area you can drag, but also increases the visual border of the window, so again you are sacrificing screen real estate which sucks.
Drag with Alt + Right-Click
You can position the cursor anywhere over the window, hold the Alt key and hold the right mouse button to resize the nearest window edge. This includes corners. The only downside here is that this requires a two hand operation. You may be able to create a custom key binding that is preferred.
The mouse binding for resizing is in
~/.config/openbox/rc.xml or whichever the appropriate rc file is in the
~/.config/openbox/ directory and looks like this:
<mousebind button="A-Right" action="Drag">