There is a hidden preference that will force links that try to open in a new window to open in a new tab instead.
Copy this and paste it into a Terminal window if you want to try it out.
Code:
defaults write com.omnigroup.OmniWeb5 OWOpenNewFramesInTabs -bool true
This is an experimental setting, which is probably why you can't toggle it on/off in the preferences yet. I found it not to my liking, as it also opens all JavaScript "window.open" windows in tabs, and usually that's not what I want (most of the time those are small windows used to do things like view the full size version of an image, or display a little dialog, or something like that).
If there was a way to
only catch regular <a href="..." target="blank"> links, I'd definitely use this feature. But give it a whirl, maybe you'll like it. If not, paste the same command into Terminal, but change "true" to "false" to turn it off again.