OmniWeb's RSS support is half-baked and half-broken. I gave up on it when I found that not all feeds work correctly with it, and after experiencing ongoing problems with the OW dock icon not updating properly after marking headlines read (the "new items" badge was usually not removed correctly after doing so, and I had to restart the browser to get rid of it), which was never resolved. Not being able to check feeds for changes any more often than once per hour was a problem too. I have a list of other complaints about the poor handling of RSS in OW, but I won't go into all that.
Just grab NetNewsWire Lite. It's freeware and it's awesome.
The beta of version 3 is available
here.