To redirect all of your requests from the old domain to the new one, I would use the following.
RewriteCond %{HTTP_HOST} ^(.*\.)?olddomain\.com [NC]
RewriteRule ^(.*)$ http://%1newdomain.com/$1 [R=301,L]
It matches both the subdomain and request URI and merges them with the new domain. If there is no subdomain, it proceeds to match the request and will redirect without a subdomain.