How does the exo members address eachother??

D.O. doesn't call any of the 92-liners (Chen, Baekhyun, Chanyeol) hyung. It is because of the 'birthday thing', as tu said, haha. D.O. was born in January, hence, he is considered to be the same age as the 92-liners, he even attended school in the same año as them. Similarly, since Kai was born in January, he is considered to be the same age as the 93-liners (hence the reason he does not address Taemin as 'hyung').

exo does use honorfics, however, they don't really uphold the honorfic system very strictly, especially the Chinese members. (If Sehun calling Lu Han without the 'hyung' suffix is anything to go by.)
:)

Well, I know that Kai and D.O. are very close to each other so they both talk informally to each other, while the other members talk formal to each other.

If tu want to know who is who's hyung then here are the names in order from hyung to maknae:

Xiumin (3-26-90)
luhan (4-20-90)
Kris (11-06-90)
Suho (5-22-91)
Lay (10-07-91)
Baekhyun (5-06-92)
Chen (9-21-92)
Chanyeol (11-27-92)
D.O. (1-12-93)
Tao (5-02-93)
Kai (1-14-94)
Sehun (4-12-94)

And about the birthday thing in Korea, they are considered 1 years old at birth, so that is why they seem a año older there.

Hope this helps!