The word ᴱQ. oksa “joint” appeared in the Qenya Lexicon of the 1910s under the early root ᴱ√OSO¹ whose other derivative was ᴱQ. oswe “hip” (QL/71).

Neo-Quenya: I’d retain the word ᴺQ. oxa “joint” for purposes of Neo-Quenya, but would not derive it from √OS any longer. I would assume it is derived from a Neo-Root ᴺ√OTH, a variant of ᴹ√OS “round” perhaps originally referring to the round joint. Compare Q. ruxa < RUTH + kā ( PE17/188).