Oleksiy

I don't recommend this name for anglophone people, as it is clearly Ukrainian, and often mispronounced by English speakers: the K and S get mixed and it becomes Oleskiy, which is a POLISH LAST NAME, and it's very frustrating for the name bearer.

This is the Ukrainian version of the popular Russian name Alexei or Aleksei. If you are in an English speaking country it might be best to just use Alexander.

I would not recommend using Alexander for Oleksiy, as these are two different names. But you could call Oleksiy just Alex, that's okay.

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Alexander is a combination of two Greek names:
Alexios = Oleksiy
+ Andros = Andrew
So if you use Alexander for Olexiy, you're basically adding Andrew to his name all of a sudden.

Oh-lek-see? For an Anglo-Saxon child, probably not a great spelling. Olexi is still "strange" to the anglophone eye, but it's at least more easily sorted out for pronunciation.