I finally had enough oregano varieties to do a proper taste test this morning. There are a surprising number of types of oregano, most of which are just called "oregano" even though the plants looks and smell different. It's not like basil, where each cultivar gets it's own unique name!
AG oregano (Origanum vulgare) - Large leaves, probably the most pure "oregano" smell and taste of the bunch. Held up well with other flavors.
Zaatar (Origanum syraicum) - Smaller leaves, much more oil in them. Had the strongest and most complex flavor, with hints of pine. Very prolific in the aerogarden.
AG marjoram (Origanum majorana) - More delicate leaves. Flavor was slightly more complex than the main oregano, but also more subtle.
I taste-tested each with a slice of tomato, salt, and balsamic vinegar. The zaatar and AG oregano flavors held up, whereas the marjoram blended and faded into the background.
For cooking I think that all of these would be interchangeable.
I got mine at the Thyme Garden Herb Co. They have a good variety of seeds, though I've also had a fair number of seeds from them that didn't germinate. When that happens I'm never sure if it's due to bad seeds, or that if something just wasn't suited to the AG.
I think za`atar can refer to either the herb or the spice mix that it's part of. It gets confusing.