Thank you, Troy. I believe I've figured it out. In Firefox, there are two kinds of bookmarks: regular and "Live". At the top of a page showing RSS feeds, they place a dialog offering "Subscribe to this feed" with a checkbox to use "Live Bookmarks". If you do, you get a "Live Bookmark". (Note: You can also set a regular bookmark to the same page, and also since this dialog never disappears, it nudges you to make more than one Live Bookmark to the same site.)
Anyhow, in OmniWeb, it appears that there is no option to explicitly make a special/RSS bookmark, but if you create a (regular) bookmark to an RSS feed, OmniWeb will automatically make it into a special "News Feed", and will download articles whenever it "Checks for changes", either on the daily/hourly schedule, or when you click "Check now". As with Firefox, downloaded articles are stored on your hard disk.
So, it seems that OmniWeb's "News Feed" bookmark is the equivalent of a Firefox "Live Bookmark", but OmniWeb is "automatic" and Firefox is "manual" (giving you a couple ways to do things you probably didn't want to do, and forever wasting 100 pixels at the top of every RSS page).
Also, I note that OmniWeb and Firefox are the only two web browsers that store articles on your hard drive. Unless Safari 4 started doing this.
If anyone knows of any errors in the above, please reply let me know.