You don't have to re install.
If it's windows 7 home on both. regardless of retail or OEM. They are the same the only difference is the product key. So just enter your OEM product key on the pc you want it on.
Keep in mind, you may only have your retail key installed on 1 machine at 1 time.
Having it on more then one is a breach in the terms of service and could end up blacklisting your product key.