A word for “mountain” in Quenya whose stem form was oront-, so that it’s plural would be oronti (Ety/Ety/ÓROT).
Conceptual Development: There were a number of competing “mountain” words in Quenya of similar derivation, all based on the root √ORO “rise”; its Sindarin cognate S. orod “mountain” was much more stable in form. The earliest iteration of these Quenya words was ᴱQ. oro “hill” in the Qenya Lexicon of the 1910s as a derivative of the early root ᴱ√ORO, unglossed but with other derivatives like ᴱQ. oro- “rise” and ᴱQ. orto- “raise” (QL/70). The word oro “hill” also appeared in the Poetic and Mythological Words of Eldarissa from this period, alongside a variant form oron(d) of the same meaning (PME/70).
The variant oron reappeared in the Declension of Nouns from the early 1930s, now with the gloss “mountain” (PE21/33); its inflected forms indicate a stem form of {orom- >>} orum- (PE21/34 and note #125). ᴹQ. oron “mountain” appeared again in The Etymologies of the 1930s as a derivative of the root ᴹ√OROT, this time with a stem form oront- as indicated by its plural oronti (Ety/ÓROT). Oron appeared once more in the name Q. Oron Oiolossë “Mount Everwhite” from the Quendi and Eldar essay of 1959-60 (WJ/403).
In Notes on Galadriel’s Song (NGS) from the late 1950s or early 1960s Tolkien gave the variant forms oro, orto “mountain” as derivatives of √ORO/RŌ “rise, mount” (PE17/63-64). ᴹQ. orto had previously appeared in The Etymologies of the 1930s as a derivative of the root ᴹ√OROT but with the gloss “mountain-top” (Ety/ÓROT). Hints of this earlier meaning can be seen in the 1968 word Q. orotinga “mountain-top” though in this compound the second element Q. inga also means “top” (VT47/28). Orto “mountain” may be the final element of the 1968 name Q. Tarmacorto “High Mountain Circle”, but more likely the last element is derivative of √KOR “round”, perhaps ✱Q. corto “circle” (NM/351).
As for oro, it meant “mountain” as an element in many late names: Q. Orocarni “Red Mountains” (MR/77), Q. Orofarnë “Mountain Ash” (PE17/83), Q. oromandi “mountain dweller[s]” (PE16/96), and Q. Pelóri “Mountain Wall” (PE17/26), though in one place Tolkien glossed the prefix oro- as “hill” (PE17/83), perhaps a callback to its meaning in the 1910s.
Neo-Quenya: For purposes of Neo-Quenya, I feel oron(t) for “mountain” is better established among Neo-Quenya writers; it is the form used in Helge Fauskanger’s NQNT (NQNT), for example. The word Q. orto was not used for “mountain” until quite late, and I would stick with its 1930s meaning “mountain-top”. As for Q. oro, I would use it as “mountain” only in compounds, not as an independent word.
oron (oront-, as in pl. oronti) noun "mountain" (ÓROT; the root occurs in orotinga, q.v.) Oron Oiolossë "Mount Everwhite" (WJ:403)