mx4.OML.RU has no NS records.
NS records for this zone are defined at OML.RU.
NS records for this zone are defined at OML.RU.
Question and response
Question
dig mx4.OML.RU. NS
Answer
The response has no answer records.
Authority
OML.RU. 822 SOA ns.megagroup.RU. dnsmaster.megagroup.RU. 1139964 21600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0