Mountain in Greek mythology sandwiched between Olympus and Pelion by Otus and Ephialtes (4)

I believe the answer is:

ossa

(Other definitions for ossa that I've seen before include "So as to turn to bones" , "famous mount" , "Hard things" , "Technical name for bones" , "So as to become a Greek mountain" .)

I've seen this clue in The Mirror.

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