[JKT]

Obviously, this is not normal behaviour. Try deleting the following files from the Application Support folder (~/Library/Application Support/OmniWeb 5/ - note, not all of these will necessarily be present as some are temp files created while OmniWeb is open):

History.plist
HistoryIndex.ox
HistoryIndex.ox.lock
HistoryIndex.ox.log

Re-launch OmniWeb and see if that solves the problem (note, you will lose your browsing history by doing this, but no other preferences or settings will be lost).

If that doesn't help, what happens if you temporarily revert to the default preferences, does this solve the problem (i.e. move the com.OmniGroup.OmniWeb5.plist file from the ~/Library/Preferences folder, re-open OmniWeb and see if you can quit it)?