The root √OKO was mentioned a couple times in Definitive Linguistic Notes (DLN) from 1959 with glosses like “wicked, evil” and “evil, bad” serving as the basis for Q. olca/S. ogol “bad, wicked, evil” along with similar words (PE14/149, 170). In other notes from this period Tolkien said √OKO “evil” influenced the meaning of the Sindarin root √AK “hostile return” (PE17/167). In one of these 1959 notes, Tolkien wrote UK, UKLA below OKO with derivatives Q. ulca and S. ogl, all unglossed (PE17/149); Q. ulca was the word Tolkien used most frequently for “evil” in Quenya. In notes from 1968, Tolkien mentioned √UK “nasty” in passing, without giving any derivatives (VT48/25); Patrick Wynne suggested this form of the root may have been connected to 1969 √UG “dislike” (VT48/32 note #15; PE22/160).
Neo-Eldarin: For purposes of Neo-Eldarin, I prefer Q. ulca for “evil”, but I think √OKO “wicked” might coexist with it as variant to salvage other words from that root.
In a 1969 essay on negation, Tolkien restored √LA as basis for the “negative of fact”, and altered the meaning of Q. ú to be “bad, uneasy, hard” as a sort of “negative with a bad sense” based on this new root √UG “dislike” (PE22/160). This is similar to the usage of these ú-forms in The Etymologies of the 1930s, where the root ᴹ√GŪ was a negative root, but its derivative ᴹQ. ú- was “not (with evil connotation)” (Ety/GŪ), though in the 1930s it seems to have been a true negative, as opposed to 1969 where it meant “difficult” or “impossible”. See the entry on the Quenya negative for a more information on the conceptual development of this and other negative roots.