Cook in it. It’s a waste to fry with an expensive olive oil. High heat can destroy subtle flavors. Any inexpensive oil rated Very Good or even Good should be fine. You could even consider a non-virgin olive oil, though we found minimal price differences between that and extra-virgin.

Drizzle it. Unlike neutral vegetable oils, olive oil imparts its own character to a dish, so think of it as a seasoning. Extra-virgin oils have nuances you won’t find elsewhere. Some people may be startled by a sharp, peppery bite from a robust oil, such as McEvoy Ranch, which stands up to red meat, salad, and cheese. The mellower Lucini, with a citrus note, might pair better with delicate fish. But there are no rules. Try a few olive oils to find what you like.

Keep it fresh. An unopened container of a high-quality olive oil may stay fresh for up to two years after it’s packed, though there will be a gradual falloff in flavor even if the bottle remains sealed. Once it’s opened, you can store olive oil, tightly capped, for months in any cool, dark place. (Heat, light, and air can degrade the taste of olive oil and possibly its nutrients.)

Oil can be refrigerated if you don’t use it often. It will liquefy quickly at room temperature. Oil will keep better in glass than in plastic, and the darker the bottle, the better. If you transfer oil from its container into a smaller bottle or dish for serving, don’t pour fresh oil on top of old, which increases the risk of rancidity.

Country of origin. Italian-sounding brands dominate store shelves, and many people assume that the best oils come from Italy. Companies play off that assumption, so read labels carefully. Bertolli’s main label, for example, says "Imported from Italy"; the back, in minuscule type, reveals that the product may contain olives from eight countries, including Italy. If heritage matters to you—and the tests show that it shouldn’t—look for clear statements such as those used by Colavita: "Obtained exclusively from olives harvested and pressed in Italy."

Acidity. You may see an acidity number on the label; the lower the number, the better the flavor. To be called extra-virgin, the Department of Agriculture says, oil must have a free-fatty-acid content, expressed as oleic acid, of no more than 0.8 percent per 100 grams. Many factors affect acidity levels, including the maturity of the fruit, harvesting and pressing practices, cleanliness of the presses, and whether the olives were picked off the ground or from the tree.

First cold press. The term harks back to a time when olives were crushed under huge stone wheels, the paste was then spread across mats, and mechanical pressure was applied to squeeze out the oil. That procedure has been replaced by more high-tech and hygienic techniques, though some producers still incorporate stones in sophisticated machinery. As for "cold," all extra-virgin olive oil comes from the first pressing of the olive paste produced through "cold" or mechanical means, without the use of heat or chemicals. The claim is meaningless.

Color. Among olive oil’s many hues: pale yellow, almost honey gold, deep yellow-green, and bright green. Color isn’t linked to quality, but it can hint at an oil’s general character. A golden hue suggests the use of ripe olives, which tends to result in nutty, buttery, or fresh, flowery notes. Green signals fruit that’s not fully mature, which may convey a sharp, bitter (in a good way) taste reminiscent of fresh-cut grass, unripe banana, tart apple, or other herbs and vegetables.