O-negative was a Human blood type.

This blood type was one of the three (the other being B-negative and AB-positive) remaining blood types Loomis was required to get for Damron, in order for the Xindi-Reptilian to synthesize a virus for their bio-weapon, which required samples of all eight types of Human blood to be properly calibrated. Detroit prostitute Georgia Tandy had a blood type of O-negative. After delivered Damron with Tandy, he requested to be paid his bounty, but was told he wouldn't be paid until they received that last two blood types. (ENT: "Carpenter Street")

After being changed into a Solid, specifically a Human, by the Great Link in 2372, Doctor Julian Bashir noted that Odo's blood type was O-negative. (DS9: "Broken Link")

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