Tolkien used similar Elvish words for “seven” for much of his life. The earliest derivation for this number appears in the Qenya Lexicon of the 1910s, where ᴱQ. otso “seven” is given under the root ᴱ√OTO “knock”, though the etymological relationship is unclear and Tolkien marked ᴱQ. otso with a “?”; the root also has the derivatives ᴱQ. otoke “beating of breasts, wailing” and ᴱQ. otto- “knock” (QL/71). G. odin “seven” from the contemporaneous Gnomish Lexicon is probably related (GL/62).
The “knock” sense may have lived on in roots like ᴹ√TON, ᴹ√TAM and √TOM (PE22/103; Ety/TAM; PE17/138), but in The Etymologies of the 1930s, words for “seven” were derived from the root ᴹ√OT with two distinct extensions ᴹ√OTOS and ᴹ√OTOK, producing respectively ᴹQ. otso and N. odog “seven” (Ety/OT). This primitive otos/otok variation reappeared in Tolkien’s writings on Elvish Hands, Fingers and Numerals from the late 1960s (VT47/42; VT48/6), though at various points Tolkien considered deriving the Quenya word from ✶otok (VT47/42) and the Sindarin word from ✶otos (RC/384; VT42/25). In his later writings, Tolkien seems to have favored ✶otos as the “true” ancient root for seven, explaining S. odog as variant produced by analogy with other forms like eneg “six” after the final s was lost (VT42/25, 31 note #61).
A root appearing in a list of “sound words” from 1959-60, described as “making a hissing foaming noise” and serving as the basis for the name Q. Ossë (PE17/138). This is only one of many different etymologies for that name, and I think it probably represents a transient idea.