I was thinking:
The original genitive (*-osyo) has been lost in the Slavic o-stems,
replaced by the Ablative *-รต:d. The Ablative itself is a contraction
of thematic vowel -o- + ending -od > -o:d. Suppose in monosyllabic
pronouns such as *tU/*kU, *sI/*jI the uncontracted form survived into
proto-Slavic as *too(d), *koo(d), *seo(d), *eo(d). We know that in
the Anlaut, o- and u- receive a euphonic prefix *w- (v-) or *g(h)-
(h-), basically to resolve a(n) hiatus with the preceding vowel (in
Common Slavic, words ended in a vowel). The same hiatus blocker would
have been inserted before o in the Ablative, giving *tovo, *kovo,
*sevo, *jevo (as in std. Russian pronunciation) or *toho, *koho,
*seho, *jeho (> togo, kogo, sego, jego).
=======================
Miguel Carrasquer Vidal
mcv@...