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What CC said. It is essentially the darkest wrapper available. Now, different manufacturers have different definitions of what makes a cigar an oscuro. La Flor Dominicana's oscuros naturals are pretty darn dark as are the JR Ultimate Oscuros. The ERDM oscuros (robusto, robusto larga, etc.) are usually not as dark, looking more like a typical maduro.
On the otherhand, I have seen some maduros that could easily be classified as oscuro, but are not called that by the manufacturer. Some Series Rs come to mind.
Now, what kind of tobacco is it? That is another question. LFD uses an Ecuadoran leaf and a "secret" aging process to come up with the oscuro natural. Many manufacturers start with a Conn Broadleaf to get their oscuros. I've seen some Costa Rican and Brazilian wrappers that I would easily qualify as oscuro.