Oregano is really a wild variety of marjoram. There are a number of wild varieties grown but the most commonly commercially available variety has small rounded olive leaves which occur oppositionally on the stem.
Choose unblemished bunches. Avoid bunches with yellowing leaves. Store covered in the crisper section of the refrigerator. Oregano is an exception to the general rule that most herbs are best fresh. Dried oregano especially when sold as dried whole bunches has a strong aromatic warm flavour that is more intensive when dried. Available all year round.