I must ask then, what is the explanation for Aleksandra-Ola??
it is a historical one - in many Polish dialects of the past 'a' sounds in certain positions developed into o's
this is certainly evident in comparison of some Polish and Czech words
krowa - krava (cow)
głowa - hlava (head)
broda - brada (beard)
the forms with a's are evidenced to be historically older so Czech retained older forms inherited from 'Common Slavic' while Polish (and similarily Russian) have introduced o's in the place of some a's
in some Polish dialects the phenomenon continued for much longer historically and has much broader scope therefore- that's why standard Polish 'ptak' (a bird) is 'ptok' in Silesian and many other Polish southern dialects like that of Górale (which sounds Górole in their speech - Górale speech also has different way of accenting words - most get accented on the initial sylable) - standard Polish 'krowa' turns into 'krowo' in Silesian as a further example
the use of the name Aleksandra and Aleksander in Polish dates back to the times that a-> o sound alteration was still active in those dialects that influenced Polish literary language - however the name Alicja was adopted later and the short name developed was Ala - no further a->o sound alteration was active by then in the standard literary Polish (as far as I know more northern dialects overtook as those that were recognized as 'proper Polish')