You cannot create a CNAME record for a Dynamic DNS hostname from Dyn, just an A, AAAA, and MX record.
However, you can check the Wildcard option for your hostname which creates a wildcard alias record:
If you do so, onlineoffice.myhostname.dyndns.org or anything.myhostname.dyndns.org will all point to myhostname.dyndns.org, just as CNAMEs would do.
Oracle support has not been of much help as they tried to explain how to set up a dyndns host in the first place.
Yes, this seems to be typical to just respond with the standard text of their knowledge base articles, without any spot on, and I also received such useless responses on support tickets. This significantly decreases their reputation in my mind...