A basic structure of an SOA record looks like:
Code:
@ IN SOA <primary-name-server> <hostmaster-email> (
<serial-number>
<time-to-refresh>
<time-to-retry>
<time-to-expire>
<minimum-TTL> )
So you would declare the primary name server in the SOA for otherdomain.com. So in your case if the responsible name servers for otherdomain.com is ns1.mysite.com then that's what you need to add in the SOA for otherdomain.com
Regards
Andy