What's the diffetrence in Ni-san and Oni-san?
Can you tell me the difference? They both seem the same to me.

Question

Answer

Both have the same meaning (older brother), but Oni-san's "O" makes it more polite.
(Oni-san can also mean "a young man.")

Answer

お兄さんは、兄さんより丁寧な表現です。
oni-san is more polite than ni-san.

Answer

oni-san presents politeness
while ni-san shows either intimate relationship or kind of rudeness