There is this parameter, CPU_COUNT that you *can* set. Although Oracle doesn't recommend it be changed - does anyone have any practical experience with it ?
Your question is one of laws and licenses, not technology.
Only with "hardware partitioning", a feature that generally appears in only very high-end (and high-price) servers, are you able to (legally) license fewer CPUs than are physically installed in the server. If your server happens to be an IBM P520, for example, you can do what you describe. With most linux hardware, you cannot (yet).
Note: you need to thoroughly understand the difference between "hardware partitioning" and "software partitioning" as Oracle defines it. The dividing line is quite hazy in places.
-- Cheers, -- Mark Brinsmead Staff DBA, The Pythian Group http://www.pythian.com/blogs