In a small saucepan, gently heat the rosemary and oil over low heat, until the aroma emerges, 3 to 4 minutes. Transfer to a heatproof glass or enamel container and allow to cool at room temperature for 2 hours, making sure the rosemary remains completely submerged. Cover well or pour into a jar and seal tightly and refrigerate for 2 days. Bring the oil back to room temperature and strain it through a double thickness of cheesecloth, then use immediately.