Apex domains cannot have CNAME records.
Question and response
Question
dig oci.oraclerealm1.com. NS
Answer
oci.oraclerealm1.com. 5 NS ns3.p200.dns.oraclecloud.net.
oci.oraclerealm1.com. 5 NS ns2.p200.dns.oraclecloud.net.
oci.oraclerealm1.com. 5 NS ns1.p200.dns.oraclecloud.net.
oci.oraclerealm1.com. 5 NS ns4.p200.dns.oraclecloud.net.
oci.oraclerealm1.com. 5 NS ns.us-phoenix-1.dns.oraclerealm1.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0