Sort olives, discarding stems, leaves, and bruised or moldy fruit; rinse olives and drain. With a knife, cut down 1 long side of each olive through to pit.
Place olives in 1-gallon or 2-quart jars (glass or plastic; 2 gal. total volume) with noncorrodible lids, filling to within 1 inch of top. Fill jars to brim with water. Partially fill pint-size heavy plastic food bags (1 for each container) with water and seal; set a bag on top of olives in each jar to keep fruit submerged (water will overflow a little). Set jars away from sunlight.
Once a day, drain water from the jars, holding olives back with your fingers. Rinse any foam or scum from jars and bags; refill the jars with cool water. (You may see a small amount of harmless bubbling in water; olives will also lose their brightness and leak color into the water.) Repeat daily (it's fine if you miss a day now and then) until olives have a mild, pleasantly bitter taste, about 5 weeks total.
Drain olives and pour salt equally over olives in jars.
Pour vinegar and 7 cups water into a 5- to 6-quart pan. Bring to a simmer over high heat. Pour hot liquid over olives to within 1/2 inch of jar rims. Let cool. Pour olive oil on top of liquid in each jar to completely cover surface, then secure lids.
S$? S(?rate olives until vinegar flavor permeates them, 2 to 3 weeks. Taste; if desired, add 1/2 cup more salt equally to jars and chill for 2 more days or up to 6 months. With time, olives become increasingly mellow. Once a month, check oil surface for scum or mold; skim off any. If olives smell fine and are firm, continue to store. If olives smell bad or become soft, discard.
To serve, remove desired quantity of olives with a slotted spoon. To season, coat lightly with olive oil and add garlic or herbs to taste (see notes).