Mate:
> I don't see why some linguists reconstruct *-o:ns in the A. pl.
> of o-stems.
Actually it makes sense more analogically than logically. The
same stems have lengthened vowel in the nominative plural.
This is obviously because of the contraction of thematic *-o-
and plural *-es, producing *-o:s. As a result, lengthening
was associated with plurality (since *o-stems are not lengthened
in the singular nominative). This was transferred to the
accusative plural since the nominative and accusative are
intrinsically paired as a "strong-case" system marking the
subject and object (the major players of a sentence put in
opposition to the indirect objects marked by the other "weak"
cases). So the same plain-versus-lengthened contrast was now
established to help mark plurality in both the nominative and
accusative.
= gLeN