Apex domains cannot have CNAME records.
Question and response
Question
dig OML.RU. NS
Answer
OML.RU. 5 NS ns1.megagroup.RU.
OML.RU. 5 NS ns2.megagroup.RU.
OML.RU. 5 NS ns.megagroup.RU.
Authority
The response has no authority records.
Additional
ns.megagroup.RU. 5 A 185.32.58.2
ns1.megagroup.RU. 5 A 185.32.57.63
ns2.megagroup.RU. 5 A 185.187.90.53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0