What is currently being displayed at "oldcompany.com"? Nothing? If so, it may deindex naturally.
A better option would be the following:
Add the following text to "robots.txt":
Code:
User-agent: * Disallow: /old/
Replace "/old/" with the directory path of "oldcompany.com", otherwise everything on your server will stop being crawled.
Furthermore, add the following to ".htaccess":
Code:
redirect 301 /old/ http://www.newcompany.com/
Again, replace "/old/" with the directory path of "oldcompany.com".
Cheers!