I agree
Olivier is better than
Oliver, but it is a little awkward to say in English. Do you put on a French accent just for the name? What exact vowel would you use at the end, e or ə or eɪ? Would you ever add a rhotic r for certain accents or would that make you sound illiterate?
Also, I wouldn't use it because I don't like Oli/Ollie/Olly.
Liv and
Livy are okay but could sound girly.
Oliver is short enough already that people won't shorten it, but
Olivier is longer and if people are confused about how to say it without a French accent, they're more likely to want to say
Ollie.