(Marcelo Del Pozo/Reuters)

The best guarantee of olive oil quality is the merchant who sells it to you. With the suppliers listed here, if you come across a defective bottle (yes, it happens), they will be quick to exchange it or refund your money.

Apart from these suppliers, your best resource is to read the label on the bottle or tin. It may not be very helpful, because it often is written in Italian, Spanish or Greek, but at least you can gain some sense of quality. If the oil is marked DOC or DO, this is a European guarantee that it has been made according to specified regulations to control quality. If it also carries an organic certification, that is a further indication of quality.

The finest olive oil is estate bottled; that is, produced from olives grown on a particular farm and bottled at the source. But that information is not always easily accessible. And the very best oils are the freshest oils; therefore, look for a harvest date of 2016, the most recent harvest in the Northern Hemisphere. A use-by date is more opaque, because it is 18 to 24 months after bottling, and most oil is not bottled when it’s produced. Thus, a use-by date of December 2017 could indicate bottling in December 2015, at which point the oil might already be a year or so from harvest.

To be avoided are olive oils that are blended from many different sources. European law requires that the source of the oil be listed on the label. So if a label says “extra-virgin olive oil from Greece, Tunisia, Italy and Spain,” don’t buy it, no matter how tempting the price. It’s cheap oil, made up of cheap oils from all over the map. It may still qualify as extra-virgin, but it isn’t high quality. The best olive oil is very expensive and should be treated accordingly: always kept in a cool, dark place — but not the refrigerator.

Some top sources for extra-virgin olive oil, including from the most recent harvest.

Olio2go

8400 Hilltop Rd., Suite H, Fairfax

866-654-6246

olio2go.com

Italian oils. Storefront is open Monday 10 a.m.-5 p.m.; Wednesday-Friday, 10 a.m.-5 p.m.; Saturday, 10 a.m.-2 p.m. Also hosts classes, tastings and events.

Gustiamo

718-860-2949

gustiamo.com

Italian oils.

Market Hall Foods

888-952-4005

markethallfoods.com

Italian, Spanish and California oils, with an especially good selection of the last.

Zingerman’s

888-636-8162

zingermans.com

Oils from all over the world.

Nancy Harmon Jenkins