Oscuro means “dark” in Spanish. In wrapper terms it is the far end of the color scale: darker than a standard maduro, sometimes almost black. The color comes from time, heat, and fermentation, along with the type of leaf used at the start, not from dye.
Because the category is informal, one factory’s oscuro may be another factory’s maduro. Treat the word as a comparison of shades. The blend inside can be mild or full; the wrapper color does not tell you which.
See maduro for the more common dark-wrapper term, and colorado for the reddish-brown shade in the middle of the range.