Combine the rosemary and oil in a 1/2-pint Mason jar, cover, and shake the jar 2 or 3 times to mix. Let the jar sit for 2 days, until the rosemary falls to the bottom. Strain the contents through a fine sieve into a 1-cup glass measuring cup. Rinse and dry the Mason jar and return the oil to the jar. Seal and store in a cool, dark place. The oil should be ready to use immediately, with a shelf life of about 2 months.