Oregano is a warm, aromatic herb used widely in Italian, Greek, and Latin American cooking. It's closely related to marjoram, but it lacks marjoram's sweetness and has a stronger, more pungent flavor. It's readily available both fresh and dried. Fresh oregano has a milder, more delicate flavor, but both forms pair extremely well with tomato-based dishes, like pasta and pizza.
Mexican oregano looks similar to Mediterranean oregano, but Mexican has a stronger resinous and earthy flavor. (The plants are from different but closely related botanical families.) While you can substitute the latter in recipes calling for Mexican oregano, it won’t impart the same robust flavor.
Supermarkets package fresh herbs in various ways: loose in small plastic boxes, fastened in bunches with rubber bands, or sometimes still growing in a pot. No matter the packaging, look for herbs with vibrant color and aroma (open up those boxes for a sniff), and avoid those that are limp or yellowing, have black spots, or don't smell totally fresh and appetizing.
Wash herbs only when you're ready to use them, because excess moisture shortens their shelf life in the refrigerator. Greenhouse herbs will be cleaner than field-grown and may not need washing. But if the herbs look or feel sandy, wash them no matter what their origin.
To wash fresh herbs, put them in a large bowl of cool water and swish them to release grit. Lift the herbs out of the water with your hands, a sieve, or a skimmer. If you see a lot of grit on the bottom of the bowl, wash the herbs again in a fresh bowl of water. Spin them dry in a salad spinner or gently blot them dry by rolling them up in a clean towel.
Chop fresh oregano with a sharp knife or snip with scissors. A sharp knife is imperative for chopping herbs. A dull one will crush and bruise tender leaves, giving you blackened rather than green results. Try using scissors to snip off small amounts.
You can refrigerate fresh oregano in a plastic bag for up to 3 days. Dried oregano should be stored in a cool, dark place for no more than 6 months.