The O/A phonetic distinction in Russian and related languages is tricky, with some dialects making a distinction and some not (https://en.wikipedia.org/wiki/Vowel_reduction_in_Russian and https://en.wikipedia.org/wiki/Akanye)

If someone would pronounce Alek and Olek the same, the chosen transcription of the shortened nickname is pretty much arbitrary.

> Olek is a short for Aleksander.

Very likely this is due to both being transliterations from Cyrillic.

This is correct.

Oleksander is Ukranian form of Aleksander.

Or the diminutive is Sasha. For reasons?