Apex domains cannot have CNAME records.
Question and response
Question
dig omc.co.at. NS
Answer
omc.co.at. 5 NS ns1.omc.co.at.
omc.co.at. 5 NS ns2.omc.co.at.
omc.co.at. 5 NS ns3.omc.co.at.
omc.co.at. 5 NS ns4.omc.co.at.
Authority
The response has no authority records.
Additional
ns1.omc.co.at. 5 A 217.13.99.227
ns2.omc.co.at. 5 A 203.56.45.4
ns3.omc.co.at. 5 A 217.13.99.66
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0