Oregano is easily grown at home in the yard or in planters, so hopefully, you have fresh oregano at your beck and call. Many stores now carry fresh oregano in the produce department. Purchased fresh oregano branches should be rich green in color and not the least bit limp.
Whether it's homegrown or purchased, oregano should be stored in a plastic bag in the refrigerator for up to 3 days.
If you place a slightly damp paper towel in the bag with the oregano and leave some air in the bag, it may extend the life up to 1 week. You may also extend the shelf life of fresh oregano by storing whole stems with leaves in a glass of water with a plastic bag loosely tented over the glass.
Fresh oregano may also be frozen. Wash and dry oregano sprigs. Strip whole leaves from stems and place in plastic bag loosely without crushing, but remove all air. Freeze and keep in a location where it will not get crushed. No need to thaw before using. You can also mix chopped leaves with a small amount of water (or puree them) and freeze in ice cube trays. Once frozen, pop out the cubes into a plastic bag and seal tightly. Use frozen oregano within 1 year.
To dry fresh oregano, tie sprigs into a bunch and hang in a cool, dark place with good ventilation. Once dried, seal tightly and store away from sunlight.
In general, dried common oregano sold in the grocery stores is actually a mixture of different varieties of oregano combined with marjoram and thyme. As with all dried herbs, dried oregano should be kept in a cool, dark place in a tightly-sealed container and used within 6 months for the freshest flavor.
It will not spoil if kept longer, but its potency will deteriorate greatly with time.