Wash the jalapeños carefully, remove the stems, and slice them into thin rings using a sharp knife.
Place garlic cloves, oregano, and peppercorns into a clean glass jar before adding the sliced peppers.
Pack the jalapeño slices into the jar, arranging them evenly so the brine can reach all pieces.
Combine vinegar, water, sugar, and salt in a saucepan, then warm until the sugar and salt dissolve completely.
Pour the warm brine over the jalapeños until they are fully submerged beneath the liquid.
Let the jar cool for several minutes before sealing tightly and transferring it to the refrigerator.
Allow the peppers to rest overnight before using them, giving the flavors time to develop and the heat to mellow slightly.