This is undoubtedly not the best way to do it, but it does give some idea of the structure.
I'm trying to get to the "How to Buy" bookmark which is in the "The Omni Group" folder in Personal Bookmarks. Bookmark folders are just bookmarks with more bookmarks within, so I grab the bookmarks from the top-level bookmark for the folder and walk the list looking for a matching name. Probably one wishing to do a lot of this would write a routine that takes a pathname, grabs its components in succession and walks the bookmark tree to the bottom. Left as an exercise for the reader :-)
Code:
tell application "OmniWeb"
set _bookmarks to bookmarks of bookmark "The Omni Group" of personal bookmarks
repeat with i from 1 to length of _bookmarks
if name of item i of _bookmarks is "How to Buy" then
set theaddress to address of item i of _bookmarks
exit repeat
end if
end repeat
tell front browser
OpenURL theaddress
end tell
end tell