It seems that in Sindarin and Noldorin wo became o, as noted by David Salo (GS/§4.151). This sound change was primarily relevant to the variant prefixes S. go- (unstressed) vs. S. gwa- (stressed) and Tolkien described this change several times in the context of that prefix:

WŌ- together. The form wŏ if stressed > wa in Eldarin ... In N we have gwa- when stressed, as in gwanur (= Q. onóro) [TOR], gwastar (see STAR), and frequently, but only in old cpds [that is, it was a frequent form but only appeared in old compounds]. The living form was go-, developed from gwa- in unstressed positions - originally mainly in verbs, but thence spreading to verbal derivatives as in gowest (see WED). In many words this had become a fixed element (The Etymologies, 1930s, Ety/WŌ).

gwa- is regularly developed from *wo > *wa > gwa when stressed in prehistoric Sindarin. go- is from *wo > gwo > go when primitively unstressed; and also from gwa- > go- when it again became unstressed (Quendi and Eldar, circa 1960, WJ/367-368).

This go-/gwa- prefixal variation dates back to Gnomish, though in the earliest conceptual stages the primitive form was ᴱ✶ŋu̯a (GL/40, 41); see the entry on how stressed [wŏ] became [wa] for further discussion. Unfortunately, finding Sindarin and Noldorin examples of this sound change outside the context of this prefix is very difficult. All the clear examples involve this prefix in some way:

• ✶gwā-ƀandina > S. govannen “met” (PE17/16, 17)
• ✶gwa-lassiē, gwa-lassa or walass(s) > S. golas “foliage” as an element of the name S. Laegolas vs. Nandorin Legolas (Let/282, PE17/84).

In particular, it isn’t clear whether [wo] > [o] occurred generally or only initially in Sindarin/Noldorin, and whether it occurred at all outside the context of the combination [gwo]. In fact, the examples S. [?] fanwos (PE17/174) and N. cadwor “shapely” (Ety/KAT) indicate that the sound change [wo] > [o] was limited in some way, though exactly how isn’t clear.

Conceptual Developments: In the Gnomish of the 1910s, this sound change seems to be part of a general trend whereby labialized velars became velars before [o], [u]: [gʷo], [kʷo] > [go], [ko]. This sound change was noted by Roman Rausch in his Historical Phonology of Goldogrin (HPG/§2.7). Some examples:

• ᴱ✶ŋguār > G. gwōr- > G. gôr “each, all” (GL/41)
• ᴱ✶qāmē [> *kwōma] > G. côma “disease, illness” vs. ᴱQ. qáme (GG/13; GL/26)

This is not relevant to Sindarin and Noldorin, because in these later conceptual stages all primitive labialized velars became labials: [gʷ], [kʷ] > [b], [p]. The only example of [w] appearing after a velar was where initial [w] became [gw], and the only examples of this happening before the vowel o involve the unstressed prefix gwo- > go- “together”.

GS/§4.151 WGHC/§66iii @@@

HPG/§2.7