Apex domains cannot have CNAME records.
Question and response
Question
dig oi.com.br. NS
Answer
oi.com.br. 5 NS venus.oi.net.br.
oi.com.br. 5 NS ns01.
oi.com.br. 5 NS ns01.oi.net.br.
Authority
The response has no authority records.
Additional
ns01.oi.net.br. 5 A 200.202.193.140
venus.oi.net.br. 5 A 187.6.211.148
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0