Rinse oregano well with water or submerge it in a bowl of water. Drain, then wrap in a clean dish towel to remove excess moisture.
Air Drying
Bunch several sprigs of oregano together and wrap the stems with a rubber band. Tie small bunches together to ensure good air circulation. Bundles that are too thick can trap moisture and slow the drying process.
Hang upside down in an open area in your house. Try to select an area that is out of direct sunlight.
Hang until the sprigs are fully dried. Drying time will vary from a few days to several days.
Dehydrator
Place oregano on a dehydrator tray.
Set the temperature to the lowest setting, around 95°F.
Keep the dehydrator going until it is fully dried. The leaves will be dry and brittle.
In the Oven
Turn your oven to its lowest setting. Turn oven off once the oven has preheated.
Place oregano on a baking sheet lined with a rack.
Place the oregano in the oven. Check after one hour. If the oregano is not fully dried (the leaves will be brittle), turn the heat back on at the oven's lowest setting. As soon as it's heated, turn the oven back off. The oregano is done when the leaves are dry and brittle.
Notes
Harvest before flowering: Oregano has the best flavor when picked just before it starts to flower. Snip the stems in the morning after the dew has dried but before the sun gets too hot.
Keep it clean: If your oregano is dusty or has dirt on the leaves, give it a gentle rinse and let it dry completely before starting the drying process. Excess moisture can lead to mold.
Avoid direct sunlight: Whether you're air drying or using a dehydrator, keep oregano out of direct sunlight to help preserve color and flavor.
Test for dryness: Oregano is ready when the leaves are dry and crumble easily between your fingers. If they still feel pliable, give them more time.
Store it right: Strip the leaves from the stems and store them whole in an airtight container away from light and heat. Crush just before using for the best flavor.