Many of the studies done on herbs use an extraction of the plant. Oils are usually used externally; tinctures are for internal use (oil of oregano is actually a tincture).
Here are recipes for a tincture and peppermint oil from Gayle Engels of the American Botanical Council:
For an herb tincture
1. Pack fresh herbs into a clean glass jar with a tight lid.
2. Fill with 80-proof vodka.
3. Cover the mouth of the jar with wax paper or plastic wrap. Close the lid tightly and shake well. Keep in a dark place for four to six weeks, shaking daily.
4. Strain and keep the liquid in a glass bottle. Label with the name and date. It will keep for years in a brown glass bottle in a dark cabinet. Add 10% glycerin to make it more palatable, or you can use glycerin if you don't want to ingest alcohol.
For peppermint oil
1. Crush or muddle fresh peppermint leaves in a glass jar with a tight lid.
2. Cover the leaves with olive or grapeseed oil. Close the jar and shake.
3. Store for three days. Strain into a bowl and discard the leaves.
4. Pack the jar with fresh leaves, pour the oil back in the jar and cover with fresh oil. Repeat until you have the desired amount.
Twenty drops of the oil should be diluted with 1.5 ounces of another oil — coconut or almond, for example — before using it topically, and with water (1 cup per five to 10 drops of oil) for mouthwash. Never put essential oils directly on your skin (they can burn).