It's acceptable unless you're talking to a Ukrainian Nazi :)
Russian name 'Aleksandr' is pronounced as Olexandr in Ukrainian (which is arguably a dialect of Russian language). It's essentially the same name.
Alexandr is rarely referred to as 'Aleks' in Russian/Ukrainian, 'Sasha' is the right shortened form (signifies familiarity and is usually used in relation to a friend/relative/close acquaintance):
https://www.tripsavvy.com/russian-nicknames-and-diminutives-1502309
And 'Oleks' is a non-existent version AFAIK.
IMO Alex is the right version for English speakers.