mail.o2.com has no NS records.
NS records for this zone are defined at o2.com.
NS records for this zone are defined at o2.com.
Question and response
Question
dig mail.o2.com. NS
Answer
The response has no answer records.
Authority
o2.com. 900 SOA n1.02.net. hostmaster.02.net. 2026022401 10800 3600 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0