-10 hp was in AD&D but not in B/X which OSE replicates.
Technically, "-10 hp" was not AD&D.
There was a rule in the DMG labeled "Zero Hit Points" (p. 82) that if you were knocked down to exactly 0 hp ("optionally as low as -3 hit points if from the same blow which brought the total to 0") you weren't dead, but unconscious. And then you would lose hit points until -10, when you died.
This rule, through misunderstandings and urban legend, morphed at some tables into the "-10hp" rule that some people played with, but it was only every supposed to deal with the edge case of being reduced to exactly zero hit points.