Apex domains cannot have CNAME records.
Question and response
Question
dig osanet.cz. NS
Answer
osanet.cz. 5 NS decsys.vsb.cz.
osanet.cz. 5 NS nsa.ces.net.
Authority
The response has no authority records.
Additional
nsa.ces.net. 5 A 195.113.144.205
decsys.vsb.cz. 5 A 158.196.149.9
nsa.ces.net. 5 AAAA 2001:718:1:1:0:0:144:205
decsys.vsb.cz. 5 AAAA 2001:718:1001:149:0:0:0:9
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0