OmniWeb 4.5, currently in beta, uses Apple's WebCore engine, also used in Safari and descended from KHTML, which is used in Konqueror. This resolves many of the standards issues seen in older versions of the browser, and has been welcomed by most in the standards community.
The following covers the previous versions of OmniWeb, maintained for historical purposes...
OmniWeb is often criticized for its lack of standards support these days. There are a lot of people, in fact, who won't use it because of this, myself included. Omni has even stated outright that they prefer to emulate Netscape 4, because they think it makes them "more compatible." This, needless to say, pisses a lot of people off.
However, there is one feature in OmniWeb that even the most die-hard standards zealot can't help but find awfully tempting: automatic spell-checking on all TEXTAREA fields, provided via the spellchecker service of Mac OS X. While this may seem inconsequential to some, it proves incredibly useful in some circles. Among other things, it's wonderful for noding, and would be even more so if only it were HTML-aware.