Queso Oaxaca is a Mexican stretched-curd cheese from the state of Oaxaca, wound into a ball of strings. In Oaxaca it is often called quesillo.
It is made from cow's milk. It melts and pulls apart like a mild mozzarella. It is a cooking cheese more than a long-aged table cheese.
History
The cheese is southern Mexican, from Oaxaca, where the ball of strings is a market staple. The method is the stretched-curd family, the same broad idea as mozzarella, adapted to local dairies and to local dishes. Stories that assign the first ball to one person are less solid than the regional fact. Oaxaca made it, uses it, and still sells it under the name quesillo.
The cheese spread through Mexican markets and into the United States with Mexican groceries and restaurants. It is now made well beyond the state. A ball from a factory in another state can be real Oaxaca-style cheese. A ball from a Oaxacan market is the reference for freshness and for the way the strings pull. The name quesillo avoids confusion with other cheeses that merely say Oaxaca on a label.
How it is made
Cow's milk is set, and the curd is heated and stretched until it is plastic and shiny, then pulled into long strings. Those strings are salted and wound into a ball. The cook or the eater unwinds them. There is no long aging and no hard rind. The cheese stays moist and mild.
The stretch is the quality test. A good ball peels into ribbons. A dry, crumbly ball was poorly stretched, poorly stored, or made as a different fresh cheese and mislabeled. It is in the pasta filata family with mozzarella and provolone. It is not in the family of cotija, which is salted and dried, or of queso fresco, which crumbles.
Taste
The taste is milky, salty, and mild, close to a fresh mozzarella with a little more salt and a stringier chew. It should taste fresh and clean. Smoke or chile, if present, was added. It is not in the plain cheese.
A sour or refrigerated-stale smell means the ball is old. Browning in a pan adds toasted flavor that the raw cheese does not have. If it tastes like a strong aged cheese, it is not this one. The mildness is the point. The dishes around it, such as a tlayuda or a filling of squash blossoms and peppers, supply the rest.
In the kitchen
Pull the strings over a tlayuda, into a quesadilla, or into a hot corn tortilla. Melt it in a queso fundido or over enchiladas when you want a stretch and a mild milk flavor. It can be the cheese in a Mexican grilled sandwich. Use it soon. It is not a pantry wheel.
Do not crumble it over elote and expect the dry salt of cotija. Do not age it. Mozzarella, especially a low-moisture piece you can shred, is the closest substitute in a melt. Asadero and Chihuahua are other Mexican melting cheeses that can cover a quesadilla. Queso fresco is the wrong way around: it is for finishing a hot dish with crumbs, not for stretching.
Around the world
Oaxaca is the home, and the rest of Mexico uses the cheese widely enough that it is a national cooking cheese rather than an obscure regional bite. In the United States and anywhere with a Mexican market, the ball of strings is a standard item next to crema and tortillas.
A recipe that says queso Oaxaca or quesillo wants this stretch. A recipe that only says Mexican melting cheese may accept Chihuahua or asadero instead. Outside those shops, low-moisture mozzarella is the grocery substitute that will melt in a similar way, with less of the salted-string texture. The ball itself is worth buying when you can, because unwinding it is part of how the cheese is used, not a garnish.