Apex domains cannot have CNAME records.
Question and response
Question
dig o2.com. NS
Answer
o2.com. 5 NS n4.02.net.
o2.com. 5 NS n2.02.net.
o2.com. 5 NS n1.02.net.
o2.com. 5 NS n3.02.net.
Authority
The response has no authority records.
Additional
n1.02.net. 5 A 82.132.128.98
n2.02.net. 5 A 82.132.128.106
n3.02.net. 5 A 82.132.128.114
n4.02.net. 5 A 82.132.128.122
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0