Tips for preserving basil in olive oil include blanching and shocking the basil before preserving it, pureeing the basil with the oil, and freezing the mixture flattened in freezer bags or in ice cube trays. Simply storing basil in olive oil is dangerous, as the low acidity of the oil and herb make the mixture susceptible to botulism, even in the refrigerator.
Blanching and shocking the basil destroys the enzymes that cause decomposition, retaining the color and scent of the fresh leaves. The process involves immersing stems of the herb in boiling water for a few seconds and then dipping the stems in an ice bath to stop the cooking. That is followed by stripping the leaves from the stem, patting the leaves dry and pureeing the basil with olive oil. The amount of oil used depends on whether the intended final product is basil-infused oil or a thicker pesto-style mixture. The last step is to spoon the puree into freezer bags, creating thin packets that ensure small pieces can be snapped off after freezing.
Another approach is to pour the basil and olive oil puree into ice cube trays to freeze. The frozen herbs can be stored right in the trays or transferred to a freezer bag or container. Either method provides the ability to have enough basil for a single meal without defrosting the entire supply.