.htaccess Mapped domains.

Hi all this is bending my mind a little so thought i would ask here for help.

old content that needs 301 redirecting is on https://www.oldsite.com

New content is on one of the subdomains of https://www.newsite.com/new-content/

When the switch to the new site comes i will be mapping the /new-contet/ to the URL https://www.oldsite.com

As far as i understand if i just put in my htaccess file in the https://www.newsite.com hosting

redirect 301 /oldpage.htm https://www.oldsite.com/newpage/

then it would be trying to redirect https://www.newsite.com/oldpage.htm rather than https://www.oldsite.com/oldpage.htm

hope this makes sense and someone can help out, it seems like it will be a fairly simple answer but any solution i come up with would be a guess.

Thanks

Sim.